fix(client): stop an inherited PATH from shadowing an explicit Path o…

[liwenjie200543]

What

StdioClientTransport.start() merged the default environment with the caller's env by plain spread. On Windows that produces a spawn env containing both the inherited PATH (from getDefaultEnvironment()) and the caller's Path when the caller spells it that way. Windows resolves environment variables case-insensitively, the inherited entry wins, and the caller's explicit value is silently discarded (issue #2859, where an explicit Path never reaches the server process).

Change

mergeSpawnEnv now drops inherited entries that differ from an override only in casing, so the caller's entry is the single value the child sees, with the casing the caller chose. On POSIX, PATH and Path are distinct variables and both survive — the platform flag is a parameter, so tests pin both modes without touching process.platform.

Verification

• Real-process check on Windows (Node + cross-spawn, no mocks): spawning node -e "process.stdout.write(process.env.Path ?? process.env.PATH)" with an env merged the old way prints the inherited value; merged the new way it prints the caller's value.
• New tests in packages/client/test/client/stdioEnvCase.test.ts: merge semantics for case-insensitive and case-sensitive modes, plus a transport-level test asserting the spawned env keeps the caller's Path. The transport test fails against the old spread on Windows (verified by stashing the fix).
• pnpm vitest run in packages/client: 921 passed, 2 skipped. tsgo typecheck, eslint and prettier clean on the touched files.
• Changeset included.

Fixes #2859

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🦋 Changeset detected

Latest commit: 394b991

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@modelcontextprotocol/client	Patch
@modelcontextprotocol/codemod	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/server-legacy	Patch
@modelcontextprotocol/server	Patch
@modelcontextprotocol/core-internal	Patch

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