fix(sse): make streaming fetch-start cap configurable

[lorenzozanee]

Summary

The streaming response-header wait remained capped at 110 seconds even when the overall fetch timeout was configured higher. Add OMNIROUTE_FETCH_START_TIMEOUT_CAP_MS to configure that streaming cap; the default remains 110 seconds, 0 disables it, and an explicit provider cap takes precedence.

Related Issues

Fixes #15504

Validation

• Change type: other (streaming timeout policy)
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR
• SonarQube is temporarily opt-in while the private project has no quota; it is not a PR gate.

Tests Added Or Updated

• tests/unit/fetch-start-timeout-policy-11526.test.ts
• npm exec --yes --package=tsx -- tsx --test tests/unit/fetch-start-timeout-policy-11526.test.ts (4 passed)
• npm exec --yes --package=tsx -- tsx --test tests/unit/runtime-timeouts.test.ts (12 passed)
• Env-doc sync, changelog integrity, Prettier, and git diff --check passed locally.

Coverage Notes

The new test covers the timeout policy function. It does not instantiate BaseExecutor or exercise a real abort timer.

Reviewer Notes

The changelog fragment is named with issue number 15504; repository convention asks for a PR-number prefix, which was unavailable when the fragment was authored. npm run lint was not run. CI and review status are unknown.