fix: show error when required tool parameters are missing

[areebahmeddd]

summary

closes #1722

• if the llm skips a required tool parameter (like large file content hitting output token limits), the tool call now fails with a clear, actionable message instead of some vague downstream error
• catches missing or nil required params in makebeforetoolcallback before the tool even runs, for both mcp tools and native function tools

ai model disclosure

used vscode copilot (claude sonnet 4.6) to understand the issue context and help generate a fix. changes were self-reviewed and verified via:

go build ./adk/...
go test ./adk/pkg/agent/...
make lint

[Copilot]

Pull request overview

This PR improves tool-call robustness in the Go ADK agent by detecting missing/nil required tool parameters early and returning a clearer, actionable error response instead of allowing opaque downstream failures (e.g., when large parameter values get dropped).

Changes:

• Added a BeforeToolCallback guard that short-circuits tool execution when required parameters are missing or nil.
• Implemented required-parameter extraction for both structured genai.Schema and opaque JSON-schema (ParametersJsonSchema) declarations.
• Added unit tests covering required-field extraction and missing/nil required-parameter detection.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
go/adk/pkg/agent/agent.go	Adds required-parameter introspection and pre-execution validation in makeBeforeToolCallback.
go/adk/pkg/agent/agent_test.go	Adds tests for JSON-schema required extraction and missing/nil required parameter detection.

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