Pre-emptive cleanup against the upstream patterns established by recent
merges (modelcontextprotocol#318 Connection abstraction, modelcontextprotocol#319 draft-spec consistency, modelcontextprotocol#303
SEP-2243 check IDs, modelcontextprotocol#321 MockServer surface). No behavior change for
scenarios — every check is still emitted; only the shape of how they're
emitted moves toward upstream's house style.

Surface
- Revert withRequestMeta / sendStatelessRequest signatures to match
  upstream byte-for-byte. The need that pushed me to widen those
  (per-request _meta.clientCapabilities for extension declaration on
  the SEP-2575 stateless wire) is now handled inside connectStateless:
  ConnectOptions.capabilities / clientInfo are folded into params._meta
  via the trailing _meta spread that withRequestMeta already supports.
  This keeps the new "extension declaration" concept on the Connection
  layer only — the lower-level helpers stay shared with every other
  scenario in the suite.
- Document that Connection.request's extraHeaders throws on the
  stateful wire instead of "ignored with a warning" — the SDK transport
  manages headers internally, so silent drop in a conformance harness
  would mask test correctness.

Layout
- src/scenarios/server/tasks-mrtr-helpers.ts moves under tasks/ as
  tasks/mrtr-helpers.ts. All 10 importers live in tasks/; the file's
  prior placement next to input-required-result-helpers.ts no longer
  applies once those scenarios moved into tasks/.
- Drop the unused AnyResult export + zod import. The Connection wrapper
  doesn't take a zod schema, so the helper was dead on arrival.
- SEP-2322 spec reference now re-exports MRTR_SPEC_REFERENCES[0] from
  input-required-result-helpers so the codebase holds one URL per SEP
  instead of two for SEP-2322.

Naming
- All 10 tasks/*.ts switch to `readonly source = { ... } as const` to
  match the convention used by caching.ts, resources.ts,
  http-standard-headers.ts. Drops the ScenarioSource import where now
  unused.
- Scenario-scope each scenario's connect-failure check ID
  (tasks-lifecycle-bootstrap, tasks-headers-bootstrap, …) instead of
  sharing tasks-session-bootstrap. Matches the upstream caching.ts
  pattern (sep-2549-caching-connection) where each scenario owns its
  own bootstrap-failure ID.

Traceability
- sep-2663.yaml gains the positive-path counterpart to the existing
  rejection requirement under Streamable HTTP: Routing Headers, so
  every sep-2663-* check ID emitted by tasks/*.ts is declared in the
  manifest.

Verification
- Conformance vitest: 270/270.
- Build: clean.
- mcpkit testconf-tasks-v2 (47 fork scenarios + 1 sentinel): pass.
- mcpkit testconf-mrtr (3 fork scenarios + 1 sentinel): pass.