feat: render a property's description as an attribute docstring

[goduni]

Description

A schema's description became a class docstring, but a property's went nowhere: the IR
carried it and every serializer dropped it. Prose the spec author wrote for a field was
invisible in the generated client.

It is now emitted as a PEP 258 attribute docstring under the field:

class Pet(BaseModel):
    id: int
    """Server-assigned identifier."""

Why this device and not the native ones:

• it is what the method classes already do for parameters and body fields, so the two
  halves of a generated package read the same way;
• it renders identically for all three serializers — pydantic would otherwise need
  Field(description=...) on every documented field, msgspec an
  Annotated[..., Meta(description=...)], and adaptix has no equivalent at all;
• it is inert at runtime. The class body just evaluates a string, so constructors,
  decoding, and the emitted JSON schema are unchanged. Editors show it on hover.

Wrapping, embedded quotes, and backslashes are handled by the existing docstring
helper, which already switches to a raw string when the prose contains a backslash.

Relation to #10

Independent, and they compose: #10 keeps a subtype's explicit re-declaration (and its
prose) in the IR, this one puts that prose in the output. Both touch the field loop in
the three render_model methods, so whichever merges second has a one-line conflict per
serializer.

Type of change

• New feature (a non-breaking change that adds functionality)
• This change requires a documentation update

Verification

• uv run pytest --cov src --cov-fail-under=80 --cov-report=term — 328 passed, 100% coverage
• uv run ruff check
• uv run ruff format --check
• uv run mypy
• all three example libraries regenerated via their regen.sh, each passing its own
  ruff check --isolated + mypy --strict gate

Note on the examples diff

The committed example libraries are regenerated here, and most of that diff predates this
change: they were already stale by 31 lines' worth of method-parameter docstrings from an
earlier release. The 5 lines under models/ are this change.

Checklist

• My code adheres to the style guidelines of this project
• I have conducted a self-review of my own code
• I have made the necessary changes to the documentation
• My changes do not generate any new warnings
• I have added tests to validate the effectiveness of my change
• I have ensured that type checking passes by running uv run mypy

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6eefcd0) to head (8aeba77).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #11   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           31        31           
  Lines         2726      2735    +9     
=========================================
+ Hits          2726      2735    +9     

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