Send the default User-Agent on the MCP endpoint probe

[RhysSullivan]

Summary

#2254 added a default User-Agent: executor to the MCP transport, but the endpoint shape probe (run when adding a server whose connection attempt fails) still sent its initialize POST, GET fallback, discover POST, and RFC 9728 protected-resource metadata GET with no User-Agent. Servers that reject such requests still failed the add flow.

The probe now sets the User-Agent on every request through its HTTP client: the configured one if present, otherwise the default. The constant and helpers move to user-agent.ts, shared by the transport and the probe.

Tests are in #2259.

Verification

• MCP plugin vitest run, typecheck; repo lint and format:check pass.

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commit: c4af5ce