What

Add a coverage-only wait_for timeout multiplier to tests/support/hermetic_transport_fixture.h so that Http2ClientHermeticTransportTest cases (and other hermetic-fixture users) can complete their SETTINGS handshake under -fprofile-arcs -ftest-coverage instrumentation. PR #1111 diagnosis confirmed all 7 Round-3 dispatcher TEST_F (and ~30 other fixture-based tests) FAILED at the wait_for(... settings_exchanged() ..., 3s) gate (5.1-5.4s observed under coverage vs 100-300ms in Debug/Release), producing 0pp delta on src/protocols/http2/http2_client.cpp for three consecutive rounds (PR #1108, PR #1109).

Change Type

• Test infrastructure (fixture only; no src/ changes; no new TEST_F)

Affected Components

• tests/support/hermetic_transport_fixture.h (introduce multiplier + apply to wait_for default and call sites)
• tests/support/hermetic_transport_fixture.cpp (if any wait_for call sites live there)
• tests/CMakeLists.txt or cmake/Coverage.cmake (define NETWORK_COVERAGE_TIMEOUT_MULTIPLIER=5 when NETWORK_COVERAGE_BUILD is ON; default 1 otherwise)

Why

• Three consecutive rounds (test(http2): add frame_injector-driven error coverage round 2 (#1106) #1108 Round 2, test(http2): stabilize coverage and expand dispatcher branches (#1106) #1109 Round 3) merged green in Debug/Release CI but produced exactly 0pp coverage delta on http2_client.cpp (still 18.75% line / 9.91% branch, byte-identical LH/LF/BRH/BRF pre and post each round).
• PR docs(coverage): diagnose http2_client 0pp coverage delta after Round 1+2+3 (#1110) #1111 diagnosis (docs/coverage/http2_client_round-4-diagnosis.md) root-caused this as a fixture-wide handshake timing trap under coverage instrumentation, not a test-quality issue.
• Without this structural fix, every future TEST_F added to exercise dispatcher branches in http2_client.cpp will repeat the 0pp pattern. Parent epic Expand unit test coverage from 40% to 80% #953 (40% -> 80% coverage) cannot progress on its single largest gap contributor while this trap stands.

Related Issues

• Part of test(http2): expand http2_client.cpp error-branch coverage using Phase 2 substrate (round 2) #1106 (Round 1/2/3 coverage push on http2_client.cpp)
• Part of Expand unit test coverage from 40% to 80% #953 (umbrella coverage epic)
• Unblocks test(grpc): expand client.cpp error-branch coverage using Phase 2 substrate (round 2) #1107 (grpc client coverage uses the same fixture pattern)
• Diagnosis source: PR docs(coverage): diagnose http2_client 0pp coverage delta after Round 1+2+3 (#1110) #1111 / closed issue test(http2): diagnose 0pp coverage delta after Round 1+2+3 (Part of #1106, #953) #1110

Where

No src/ changes. No new TEST_F. TEST_F additions are #1106 / #1107 territory and must wait until this fix lands and a measured coverage delta is verified.

How

Technical Approach

1. In tests/support/hermetic_transport_fixture.h, add near the top of the header:

   #ifndef NETWORK_COVERAGE_TIMEOUT_MULTIPLIER
   #define NETWORK_COVERAGE_TIMEOUT_MULTIPLIER 1
   #endif

2. Update the wait_for default-argument timeout to scale with the multiplier:

   std::chrono::milliseconds timeout = std::chrono::seconds(2) * NETWORK_COVERAGE_TIMEOUT_MULTIPLIER

3. At every explicit wait_for(..., 3s) (or other literal-second) call site in the fixture and helpers (make_connected_client, etc.), multiply by NETWORK_COVERAGE_TIMEOUT_MULTIPLIER so the budget becomes 3s * 5 = 15s under coverage builds and stays 3s otherwise.
4. In the build system, set NETWORK_COVERAGE_TIMEOUT_MULTIPLIER=5 when NETWORK_COVERAGE_BUILD is ON. Use the same target-scope compile definition mechanism that already gates the existing NETWORK_COVERAGE_BUILD macro so the multiplier is target-private to test binaries.
5. Do NOT add or modify any TEST_F in this PR.

Acceptance Criteria

• Macro NETWORK_COVERAGE_TIMEOUT_MULTIPLIER exists in tests/support/hermetic_transport_fixture.h, defaults to 1, set to 5 only under NETWORK_COVERAGE_BUILD.
• Every wait_for(...) call site reachable from Http2ClientHermeticTransportTest (default-argument and explicit) scales its timeout by the multiplier.
• Debug and Release matrix CI on the PR pass (no behaviour change off coverage; multiplier is 1).
• Post-merge coverage workflow run (full lcov pipeline) on develop measures delta >= +20pp line AND >= +10pp branch on src/protocols/http2/http2_client.cpp versus the pre-merge sha. Reference baseline: 18.75% line (108/576), 9.91% branch (97/979) from PR test(http2): stabilize coverage and expand dispatcher branches (#1106) #1109 (sha d537864).
• Post-merge coverage workflow run shows the previously-failing fixture sentinel (Http2ClientHermeticTransportTest.ConnectCompletesSettingsExchangeWithMockPeer) and the 7 Round-3 dispatcher TEST_F now PASS under instrumentation (per Run tests step stdout).
• No new TEST_F introduced in this PR (zero additions to tests/unit/http2_client_branch_test.cpp and any other unit-test file).

Test Plan for Reviewers

1. Inspect the wait_for call-site diff and confirm every literal-second budget scales with NETWORK_COVERAGE_TIMEOUT_MULTIPLIER.
2. Verify the build-system change is scoped to test targets (not src/).
3. After merge, trigger the coverage workflow on develop and check coverage_filtered.info for the post-merge LH/LF/BRH/BRF on src/protocols/http2/http2_client.cpp against the >= +20pp / >= +10pp gate.

Breaking Changes

None. Multiplier defaults to 1 outside coverage builds; observable behaviour in Debug/Release is unchanged.

Rollback

git revert is sufficient. Test infrastructure only; no production-source or schema changes.

Risks

• The 7 dispatcher TEST_F may add 50+ s combined to the coverage workflow runtime when they actually complete (vs aborting at 5s today). Diagnosis explicitly accepts this trade-off.
• If the post-merge coverage delta does not clear the >= +20pp / >= +10pp gate, do NOT close this issue or claim Round 5 success; reopen the diagnosis to investigate whether more call sites need the multiplier or whether Option C (friend-injected process_frame) is needed instead.