Fix: Deep-copy session for single_turn and explicitly set branch on injected user events (issue #5686)

[9chait9]

Fixes #5686

This PR addresses an issue where sequential calls to ctx.run_node() with single_turn agents fail to receive their node_input. The fix involves:

1. Deep-copying the session and clearing events: In prepare_llm_agent_context, the session copy is changed from shallow to deep, and the session.events list is explicitly cleared for single_turn agents. This ensures each single_turn agent starts with an isolated and clean event history.
2. Explicitly setting branch for injected user events: In prepare_llm_agent_input, the user_event.branch is explicitly set to ctx._invocation_context.branch. This ensures the node_input is correctly scoped to the child agent's branch, preventing it from being filtered out by _is_event_belongs_to_branch.

[adk-bot]

Response from ADK Triaging Agent

Hello @9chait9, thank you for submitting this PR to fix #5686!

To help our reviewers process this change more efficiently, please ensure the following sections from our contribution guidelines are addressed:

1. Testing Plan: Please include a testing plan section in your PR description outlining how the changes are tested.
2. Unit Tests: Since this is a core bug fix, please add or update unit tests under tests/unittests/ to cover these scenarios and prevent future regressions, and include a summary of passed pytest results.
3. Logs / Screenshots: Please provide verification logs or console outputs showcasing that sequential ctx.run_node calls now succeed as expected.

Thank you for your valuable contribution!