docs(errors): document DBusFastError and InternalError

[bluetoothbot]

What

Add Sphinx autoclass entries for DBusFastError and InternalError in docs/source/errors.rst.

Why

Two exception classes landed today without doc coverage:

• DBusFastError (feat(errors): add DBusFastError common base class #634) — common base class for every exception raised by dbus-fast
• InternalError (feat(errors): add InternalError so library bugs are catchable #638) — signals a bug inside dbus-fast itself

They're already exported from dbus_fast.__init__, but the published API reference still listed only the older classes, so users can't discover them from the docs.

How

Two autoclass directives matching the surrounding style:

• DBusFastError placed first, before DBusError, since it's the new root of the hierarchy.
• InternalError placed next to AuthError, alongside the other non-builtin-rooted errors.

Docstrings already exist on both classes, so autodoc pulls the descriptions automatically.

Testing

Sphinx isn't installed locally — relying on CI / RTD build to confirm the directives render. The pattern is identical to the existing entries in the same file, so the risk of a syntax error is minimal.

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Quality Report

Changes: 1 file changed, 2 insertions(+)

Code scan: clean

Tests: failed (FAILED)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.25%. Comparing base (f0cbf2e) to head (32d3d48).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #656   +/-   ##
=======================================
  Coverage   89.25%   89.25%           
=======================================
  Files          29       29           
  Lines        3491     3491           
  Branches      601      601           
=======================================
  Hits         3116     3116           
  Misses        226      226           
  Partials      149      149           

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[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

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_{Comparing koan/docs-exception-base-classes (32d3d48) with main (f0cbf2e)¹}

Footnotes

1. No successful run was found on main (c7091bf) during the generation of this report, so f0cbf2e was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩