fix(mcp-adapters): preserve timeout from RunnableConfig in MCP tool calls

[pawel-twardziak]

The timeout value from RunnableConfig was being lost when calling MCP tools because ensureConfig() converts it to an AbortSignal and removes the numeric timeout field. This prevented the MCP SDK from receiving explicit timeout values.

Changes:

• Store original timeout in metadata.__lc_originalTimeoutMs in ensureConfig()
• Retrieve and pass numeric timeout to MCP SDK RequestOptions in _callTool()
• Add integration test verifying per-call timeout is properly honored

This ensures MCP tools respect timeouts passed through RunnableConfig, allowing callers to override default timeouts on a per-call basis.

Why deleting timeout is necessary (as it is so far)

• Idempotent normalization: ensureConfig may be called multiple times down the stack. If timeout remains, each call would synthesize new timeout signals and combine them, changing the effective timeout unpredictably.
• Single enforcement path: downstream code relies on signal to enforce cancellation. Leaving timeout means two competing mechanisms (numeric timeout and signal) can be applied, sometimes with different semantics.
• Propagation to children: pickRunnableConfigKeys would keep forwarding timeout to nested runnables, causing repeated re-normalization and stacked timeouts.
• Backward compatibility: a lot of components and tests assume ensureConfig removes timeout post-normalization; changing that would be a breaking change.

If an integration needs the numeric value later (e.g., to pass to an SDK that expects timeout), preserve it out-of-band (e.g., via metadata) and still use signal for actual cancellation.

Fixes #9136

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