docs(ci): document e2e-azure OIDC federated credential setup

[yeongseon]

Summary

Closes part of #183 (documentation only — items 1-4 require Azure portal access).

Adds maintainer-facing documentation for the OIDC federated credential required by the e2e-azure workflow's Azure login step. The repo rename to add the -python suffix invalidated the existing federated credential, producing AADSTS700213: No matching federated identity record found on tag pushes.

Changes

docs/testing.md

• Fixed stale trigger text in the Real Azure E2E Tests > Workflow section ("weekly schedule (Mondays 02:00 UTC)" → "Tag push (v*) or manual (workflow_dispatch)").
• Clarified that AZURE_CLIENT_ID / AZURE_TENANT_ID / AZURE_SUBSCRIPTION_ID come from the azure-e2e GitHub Environment, while AZURE_LOCATION is read from the vars context with a fallback to koreacentral.
• Added new Federated Credential (OIDC) Setup subsection documenting:
  • The exact OIDC subject: repo:yeongseon/azure-functions-validation-python:environment:azure-e2e.
  • Subject composition (repo:<owner>/<repo>:environment:<environment_name>) with explicit note that <owner>/<repo> is the GitHub repo slug — not the PyPI package name (azure-functions-validation) or the Python import name (azure_functions_validation).
  • Case-sensitivity warning and rename failure mode.
  • Reference to GitHub OIDC and Microsoft federated identity docs.
• Added new Troubleshooting > AADSTS700213 subsection with typical causes (rename, environment change, wrong app registration) and recovery steps.

.github/workflows/e2e-azure.yml

• Added inline maintainer comment block above environment: azure-e2e in the deploy_and_test job stating the expected OIDC subject and pointing to docs/testing.md. Placed near environment: (not near azure/login) because the environment declaration is what drives the OIDC sub claim.
• Added a one-line cross-reference comment above environment: azure-e2e in the cleanup job.

What this PR does NOT do

This PR addresses only acceptance item #5 of #183 ("Add a short maintainer note documenting the expected OIDC subject"). Items 1–4 require Azure-side action that cannot be performed from code:

• (Scaffold package structure and pyproject #1) Confirm the Azure AD app registration referenced by AZURE_CLIENT_ID.
• (Add repo tooling and local dev config #2) Add or update the federated credential to subject repo:yeongseon/azure-functions-validation-python:environment:azure-e2e (issuer https://token.actions.githubusercontent.com, audience api://AzureADTokenExchange).
• (Set up CI workflows and coverage reporting #3) Re-run e2e-azure.yml and confirm Azure login (OIDC) succeeds.
• (Scaffold documentation and project metadata #4) Verify the workflow proceeds to resource creation / deploy.

After the maintainer applies the Azure-side fix and re-runs the workflow, this issue can be closed.

Validation

• python3 -c "import yaml; yaml.safe_load(open('.github/workflows/e2e-azure.yml'))" → OK.
• mkdocs.yml already includes docs/testing.md under Contributing → Testing, so the new subsections are discoverable in the published docs without nav changes.

Review process

• Oracle design review (full project context preserved across review and verification turns).
• Oracle implementation review fixed one inaccuracy (AZURE_LOCATION source) before commit.