Remove npx dependency from OpenAI Agents tests

[brianstrauch]

Summary:

• replace the external MCP subprocess in the no-worker test with the existing in-process tracking server
• use a streamable HTTP MCP server in public examples
• remove stale Node prerequisites from contributor documentation

Verification:

• make sync
• make lint
• make test (214 passed, 16 skipped)
• tooling tests (70 passed)
• repository conventions
• wheel and sdist checks

[DABH]

Merging this first means #6's merge, and every later re-sync of these regions, conflicts.

Two options I see: (1) open the same three-file change against sdk-python (or fold it into #1793) and pick it up here by re-sync; (2) or, if we want this repo to own openai_agents from now on, make that explicit by dropping upstream from plugin.toml and retiring the rule in AGENTS.md rather than bypassing it. Maybe prefer the first while #1793 is still open? Wdyt?