fix(flows): inject transfer_to_agent tool for HITL confirmation resume

Closes: #5633

Problem:
When using HITL (human-in-the-loop) confirmation for transfer_to_agent, the tool was not present in tools_dict during confirmation resume, causing a ValueError and breaking the transfer flow.

Solution:

• Ensures transfer_to_agent is injected into tools_dict during confirmation resume, mirroring the logic in the agent transfer processor.
• Adds unit tests for approve, reject, and no-sub-agent paths to verify correct behavior.

Testing Plan:

• Added unit tests for all confirmation paths (approve, reject, no sub_agents).
• All unit tests pass locally: pytest test_request_confirmation.py -v
• No regressions in request_confirmation logic.

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.

Additional context:
No API or breaking changes. Fix is isolated to HITL confirmation resume logic.