Summary

Summary / capstone PR for the streaming-session correctness work. Two roles:

1. Index — the PR chain table below catalogues every streaming-session fix landed over the past week, with one-line descriptions and pointers to the PRs that carry the full analyses (Streaming session: fix retract tail leak via _free_tail #22862 for tail free, streaming session: trim spec v2 overshoot in cache_finished_req #22897 for overshoot trim + a complete appendix on spec v2 invariants).
2. Lands the last code fix + the test matrix — spec v2 bonus-slot accounting (the only correctness piece not already in main), removes the spec_v2 + streaming session incompatibility gate, and adds end-to-end test coverage across all meaningful configuration combinations.

Read this PR if you want the overview; follow the links to #22862 / #22897 for the root analyses.

Dependent PR chain

Bonus accounting fix (this PR)

Spec v2 (overlap + EAGLE) bumps kv_committed_len post-facto in _resolve_spec_overlap_token_ids:

committed += accept_lens   # accept_lens = num_drafts + 1 bonus

Normal decode does the +1 upfront in prepare_for_decode (claims the pending-bonus slot before the forward runs). Spec v2 doing it post-facto leaves a one-token race: on the finishing round, cache_finished_req fires before the next iter's resolve catches up, so save_from_req captures committed = origin + finished_len - 1. Next turn inherits one token short, and the EagleV2 inheritance test fails by 1.

Fix mirrors the normal-decode pattern:

• EagleDraftInput.prepare_for_decode: r.kv_committed_len += 1 (pre-claim bonus slot)
• _resolve_spec_overlap_token_ids: req.kv_committed_len += accept_lens[i] - 1 (subtract the upfront claim)

Net per round is unchanged (+= accept_lens); the timing moves from post-facto to upfront, so cache_finished_req always sees the post-extra-round committed value.

Also removes the spec v2 + streaming-session incompatibility gate in tokenizer_communicator_mixin.py. The accounting bugs that motivated the gate are fixed by the chain above (#22790, #22862, #22897, #22900, and this PR); spec v2 + streaming + retract now passes under strict mem check.

Test coverage

What this PR adds

12 test classes across two files, pruned by strict-superset coverage — each retract variant already exercises the non-retract path; page=256 tests subsume page=1 (since page-aligned free / allocator paths only activate at page>1).

All use SGLANG_ENABLE_STRICT_MEM_CHECK_DURING_BUSY=2 so any KV accounting drift fails the test.

Per-class shared methods

Every subclass of TestStreamingSession inherits and runs:

• test_kv_cache_inheritance — verify cached_tokens carries forward correctly across turns (skipped on page > 1 classes; constant offset doesn't fit when cached_tokens rounds to page boundaries)
• test_leak_logprob_concurrent — concurrent multi-session with logprob, watch for KV leak
• test_stress_concurrent_sessions — high-concurrency streaming + non-streaming
• test_preabort_recovery — pre-abort (rejected by create_req) doesn't corrupt session
• test_first_mid_abort_recovery — abort the very first request mid-decode, recovery starts fresh
• test_nth_mid_abort_recovery — abort an nth request mid-decode, recovery rolls back to last successful