fix(session/vertexai): preserve FunctionCall and FunctionResponse IDs in deserialization

[nuthalapativarun]

Link to Issue or Description of Change

• Closes: Vertex AI session service drops FunctionCall.ID and FunctionResponse.ID during deserialization, causing rearrangeEventsForFunctionResponsesInHistory to silently drop events #679

Description

aiplatformToGenaiContent was not populating the ID field when converting aiplatformpb.FunctionCall and aiplatformpb.FunctionResponse to their genai equivalents. The write path (createAiplatformpbContent) correctly serialized the ID, but it was silently dropped on read.

Root cause: In session/vertexai/vertexai_client.go, the struct literals for genai.FunctionCall and genai.FunctionResponse in aiplatformToGenaiContent were missing the ID field:

// Before (missing ID):
part.FunctionCall = &genai.FunctionCall{
    Name: v.FunctionCall.Name,
    Args: argsMap,
}

// After:
part.FunctionCall = &genai.FunctionCall{
    ID:   v.FunctionCall.Id,
    Name: v.FunctionCall.Name,
    Args: argsMap,
}

Impact: rearrangeEventsForFunctionResponsesInHistory uses the ID field to match function calls with their responses. With all IDs being empty strings, it mapped every call and response to the same "" key, silently keeping only the last function response and dropping all others from req.Contents in cross-invocation sessions.

Testing Plan

Unit Tests:

• Added TestAiplatformToGenaiContent_PreservesFunctionCallAndResponseIDs in session/vertexai/vertexai_test.go verifying that the ID field is correctly preserved for both FunctionCall and FunctionResponse parts after deserialization.
• All unit tests pass locally.

$ go test ./session/vertexai/...
ok  	google.golang.org/adk/session/vertexai	34.766s

Manual E2E Tests:

The bug is deterministic and fully covered by the unit test. Any multi-invocation session with tool calls would previously drop all but the last function response from history; after this fix the IDs are preserved and matching works correctly.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.

[gemini-code-assist]

Code Review

This pull request updates the conversion logic in vertexai_client.go to include the ID field for FunctionCall and FunctionResponse parts. A new test case was added to verify this mapping. The reviewer suggested refactoring the test into a table-driven format to match the existing codebase style and expanding it to cover additional fields.

[indurireddy-TF]

Hi @nuthalapativarun The branch is currently out of date. Could you please update it ?