fix(docs): CD docs build fails in mkdocstrings source highlight on api/auth.md

[wphillipmoore-vergil-agent]

Summary

The CD docs job fails at mkdocs build (via mike deploy) rendering
api/auth.md, crashing in the mkdocstrings-python source highlighter:

ERROR - Error reading page 'api/auth.md': 'NoneType' object has no attribute 'replace'
  File ".../mkdocstrings_handlers/python/_internal/handler.py", line 264, in render
  File ".../mkdocstrings_handlers/python/templates/material/_base/class.html.jinja", line 272, in block 'source'
  File ".../mkdocstrings/_internal/handlers/rendering.py", line 120, in highlight
AttributeError: 'NoneType' object has no attribute 'replace'
error: Command '['mkdocs', 'build', '--clean', '--config-file', '.../mike-mkdocs....yml']' returned non-zero exit status 1

Context

• Not caused by the config-refresh PR (chore(tooling): refresh managed config + source credentials from env #511). This is a latent docs-build
  breakage that the CD startup_failure (item 7, fixed in chore(tooling): refresh managed config + source credentials from env #511) had been
  masking since 2026-05-27. Fixing item 7 un-blocked the docs job, which now
  actually runs mkdocs build and fails.
• Python-specific: go/rust CD docs deploys succeed; they don't autodoc
  Python source via mkdocstrings.

Locked docs toolchain (uv.lock)

• mkdocs 1.6.1, mkdocs-material 9.7.3
• mkdocstrings 1.0.3, mkdocstrings-python 2.0.3
• pymdown-extensions 10.21, pygments 2.20.0

pyproject.toml pins only mkdocs-material and mkdocstrings[python]
(unversioned), so uv resolved the latest majors (mkdocstrings 1.0,
mkdocstrings-python 2.0). The crash is in the source block highlight, i.e.
show_source rendering of an object documented in api/auth.md
(pymqrest.auth).

Plan

1. Investigate root cause first (no changes yet): reproduce locally, and
   determine exactly why highlight() receives None for api/auth.md — a
   specific object/docstring construct vs. a mkdocstrings-python 2.0 regression.
2. Choose the fix from findings: pin the docs toolchain to the last building
   version (preferred if it's a regression), adjust mkdocstrings options, or fix
   a source construct — and report upstream if it's a library bug.

Acceptance

• CD docs job builds and deploys cleanly on develop.
• Fix validated (docs build green) before merge.

References

• Exposed by: mq-rest-admin-python#510 → PR #511 (item 7 secrets: inherit)
• Fleet epic: mq-rest-admin-project/.github#14

[wphillipmoore-vergil-agent]

Post-merge verification: the develop cd.yml run succeeded with docs / docs: success — mkdocstrings source-highlight crash is gone and docs deploy cleanly (pymdown-extensions>=10.21.3 on CVE-safe pygments 2.20.0). develop at d7e10b5.