fix: branch filtering in ParallelAgent subagents

[jkumz]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: buildContentsCurrentTurnContextOnly ignores branch filtering, breaking IncludeContents="none"in ParallelAgent #719

2. Or, if no issue exists, describe the change:

N/A

Testing Plan

Please describe the tests that you ran to verify your changes. This is required
for all PRs that are not small documentation or typo fixes.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally. (NOTE: Unit tests that were already failing prior to my changes are still failing - not affected by these. changes)

Please include a summary of passed go test results.

A new test suite (TestContentsRequestProcessor_IncludeContentsNone_ParallelBranch) was added to contents_processor_test.go and passes locally. Specifically, this verifies two complex edge cases for parallel interactions:

1. FirstModelCall_AlphaFCPresent: Verifies that if sibling agent A yields a FunctionCall on its branch, agent B still correctly anchors onto the root user message without being interrupted.
2. SecondModelCall_BetaFCFRPresent: Verifies that agent B successfully scans backward past sibling agent A's events to retrieve its own context (FunctionCall and FunctionResponse) as well as the root user message to prevent the empty-context crash.

Manual End-to-End (E2E) Tests:

Please provide instructions on how to manually test your changes, including any
necessary setup or configuration. Please provide logs or screenshots to help
reviewers better understand the fix.

N/A

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• 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.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

N/A