fix(delegation): keep credential pools endpoint-coherent

[oferlaor]

Summary

This current-main patch extends the provider-coherence fix from NousResearch#39862 to cover same-provider endpoint mismatches.

• reuses credential_pool_matches_provider(..., base_url=...) so named custom pools remain supported;
• requires production credential-pool entries to match the delegated child’s active endpoint before pool sharing or lease binding;
• prevents an openai-api pool for https://api.openai.com/v1 from overwriting a child configured for an Azure OpenAI endpoint;
• revalidates the pool immediately before lease binding as a defense-in-depth boundary;
• preserves legacy unscoped pool adapters;
• adds Azure/public-OpenAI mismatch and named-custom-pool regressions.

Once merged into fix/delegation-runtime-pool-coherence-pr, the existing upstream PR NousResearch#39862 will update automatically.

Validation

• python -m pytest tests/tools/test_delegate.py -q -o 'addopts=' → 161 passed
• Ruff on changed files → passed
• Python compilation → passed
• git diff --check → passed
• Production CredentialPool probe:
  • Azure child + public OpenAI pool → rejected
  • Azure child + matching Azure pool → shared