fix(server): allow JSON Accept in JSON response mode

[Genmin]

Summary

• allow Streamable HTTP JSON response mode to accept Accept: application/json without also requiring text/event-stream
• accept wildcard media ranges such as */* through a small Accept parser instead of substring-only checks
• keep the existing stricter application/json + text/event-stream requirement for SSE response mode

Context

Fixes #1944. When enableJsonResponse: true, POST requests resolve to plain JSON responses and never open the SSE stream path, so requiring clients to advertise text/event-stream rejects valid JSON-RPC-over-HTTP clients.

Validation

• npx vitest run test/server/streamableHttp.test.ts -t "JSON response mode"
• npx vitest run test/server/streamableHttp.test.ts -t "application/json-only Accept"
• npx vitest run test/server/streamableHttp.test.ts
• npm run typecheck -- --pretty false
• npx prettier --check src/server/webStandardStreamableHttp.ts test/server/streamableHttp.test.ts .changeset/json-accept-json-response.md
• git diff --check

[changeset-bot]

🦋 Changeset detected

Latest commit: 3b37516

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commit: 3b37516