fix(tools): skip credential pool inheritance when override_base_url differs

[nsyring]

What does this PR do?

Fixes a bug where subagents using a different API endpoint (override_base_url) incorrectly inherited the parent agent's credential pool, causing 401 authentication failures.

When delegating to a provider with a different base URL (e.g., parent uses OpenRouter, subagent uses Mistral), the subagent should use its own credentials, not the parent's pool.

Related Issue

No existing issue.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Changes Made

• tools/delegate_tool.py: Add URL comparison guard in _build_child_agent — skip pool inheritance when override_base_url differs from parent's base URL. Trailing-slash normalization prevents false mismatches.
• tests/tools/test_delegation_pool.py: 4 tests covering same-URL inheritance, different-URL skip, trailing-slash normalization, and empty override.

How to Test

1. Configure two providers with different base URLs (e.g., OpenRouter + Mistral)
2. Delegate a task from one to the other
3. The subagent should authenticate with its own credentials, not the parent's
4. Run pytest tests/tools/test_delegation_pool.py -v — all 4 tests pass

Checklist

• I have read the Contributing Guide
• My commits follow the Conventional Commits format
• I have searched for existing PRs to avoid duplicates
• This PR contains only related changes
• pytest tests/ -q passes
• I have added tests for my changes
• I have tested on: Debian 13 (LXC, amd64)
• N/A — no new docs needed
• N/A — no config key changes
• N/A — no architecture changes
• Cross-platform: no platform-specific code
• N/A — no tool schema changes

[alt-glitch]

Related to #8797 and #13752 — all three fix credential pool inheritance overriding delegation base_url. Also addresses root cause from #7833.