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