Fix custom-header schema freshness

[jstar0]

Summary

Make the positive http-custom-headers mock advertise a fresh tools/list schema before requiring clients to mirror x-mcp-header parameters into Mcp-Param-* request headers.

Root Cause / Context

http-custom-headers returned tools/list with ttlMs: 0, which marks the tool schema immediately stale. The scenario then required the following tools/call to include Mcp-Param-* headers derived from that schema.

A strict client that composes the custom-header and caching rules can reasonably treat that schema as stale and omit schema-derived headers. That makes the positive custom-header scenario fail because of the mock's cache hint rather than the client's header support.

Changes

• Change the positive custom-header tools/list response to use a positive TTL.
• Add a regression test that only sends schema-derived Mcp-Param-* headers when the listed schema is fresh, then asserts all custom-header checks pass.
• Update the 2026 result-field test to pin this scenario to a positive TTL while still checking the required cache fields.

Scope / Risk

This only changes the positive http-custom-headers client scenario. The invalid-tool scenario and other cacheable mock responses are unchanged.

Verification

npm test -- src/scenarios/client/http-custom-headers.test.ts -t "serves a fresh tools/list TTL"
npm run typecheck
npm run lint
npm run build
npm test

All commands passed.

Closes #354

[pcarleton]

Thanks — taking this because ttlMs: 300000 is a more realistic fixture, but wanted to note that since this PR was opened clients should not drop Mcp-Param-* on a stale schema: since modelcontextprotocol/modelcontextprotocol#2972 and #3071 the 2026-07-28 text makes header emission independent of TTL (streamable-http 'Client Behavior' steps 3–5), so a client that omitted them at ttlMs: 0 would still be non-conformant.

[pkg-pr-new]

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commit: 93dcef3