Cover the x-mcp-header and modern HTTP entry families: 19 tests

The SEP-2243 header derivation pipeline gets full coverage: static
definition validation with per-tool eviction and the logged warning (RFC
9110 token rule, control characters, case-insensitive duplicates, the
number type the spec now forbids, items/nested-properties reachability),
the base64 sentinel encoding both ways including the collision-escape row
from the spec's own table, null/absent argument omission, and the
Mcp-Method/Mcp-Name mismatch rejections (400, -32020). The known
server-side gap - Mcp-Param-* values are not validated against the body -
is pinned as a divergence carrying issue=L110 with the recognized-header
judgement call recorded so the fix re-pins under either shape.

The modern entry itself: response modes, lazy SSE upgrade, cacheable
stamping, disconnect cancellation, header validation arms, and the
initialize-removed rejections.

28 entries minted (23 tested, 5 deferred), one flip, and the ledger riders:
issue=L109 on the three embed-gate divergences, issue=L107 on the push-API
pin. 876 -> 909 collected cells, all accounted; suite green three runs.

Author: maxisbey