Studio: harden stdio MCP gating and fix transport edge cases (#5892)

* Studio: harden stdio MCP gating and fix transport edge cases

- Gate the Data Recipe stdio path behind UNSLOTH_STUDIO_ALLOW_STDIO_MCP so a hosted deployment cannot spawn local processes through recipes
- Enforce the gate inside _client() so the transport sink cannot spawn when disabled
- keep_alive=False so stdio probes/calls do not leave orphan subprocesses
- Force OAuth off for stdio servers on create and update
- Drop stored headers when a server switches transport type
- Reject a command whose first token is a URL scheme
- Add MCP gate and improvement tests

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* tests: skip Data Recipe stdio tests when data_designer is absent

The data_designer plugin is only installed in the Studio test job, so guard
the two build_mcp_providers tests with importorskip so the core matrix skips
them instead of failing on ModuleNotFoundError.

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Co-authored-by: Daniel Han <michaelhan2050@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>