feat: expose RunnableConfig metadata to search providers

[March-77]

Description

Expose RunnableConfig metadata to search providers through SearchRequest.getContext().getExtensions().

Search providers can now read runtime values such as a knowledge-base ID, per-agent topK, and score thresholds without adding them to the model-visible tool schema.

Root cause

BaseSearchCodeactTool received the framework ToolContext, but buildSearchRequest ignored it. As a result, the RunnableConfig metadata already injected by the agent framework was lost before the request reached SearchProvider.

Changes

• Copy all RunnableConfig metadata into SearchContext.extensions while building a search request.
• Use a defensive map copy so providers cannot mutate the config's metadata map.
• Preserve existing behavior when no RunnableConfig or ToolContext is present.
• Add tests for both configured and missing runtime context.

Validation

JAVA_HOME=/opt/homebrew/opt/openjdk@17 mvn clean test
9/9 Maven modules: SUCCESS
Tests run: 47, Failures: 0, Errors: 0, Skipped: 0

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Fixes #59