fix(adk): avoid cancel context races in concurrent ChatModelAgent runs

[LPX-E5BD8]

Fixes #1177

concurrent Run on one ChatModelAgent races on reactConfig.cancelCtx.
copy the config per run before writing cancelCtx.

[CLAassistant]

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[Copilot]

Pull request overview

This PR fixes a data race when a long-lived ChatModelAgent is used concurrently on the tools/ReAct execution path by ensuring per-run cancellation state is written to a per-run config copy rather than mutating a shared reactConfig / modelWrapperConf.

Changes:

• Copy reactConfig (and modelWrapperConf when present) inside each Run closure before injecting cancelCtx, avoiding shared mutable state across concurrent runs.
• Apply the same per-run config copy approach to both message-based and agentic ReAct run functions.
• Add a concurrent regression test that exercises concurrent Run calls on a shared agent with tools enabled.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
adk/chatmodel.go	Avoids concurrent mutation of shared ReAct config by shallow-copying configs per run before setting cancel state.
adk/chatmodel_concurrent_test.go	Adds a concurrency regression test covering shared-agent concurrent runs on the tools/ReAct path.

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