fix: fall back to schema key when two schemas share a title

Summary

When two component schemas declare the same title, the second one was
silently dropped along with every endpoint that referenced it. This PR
makes the second variant fall back to a class name derived from its
schema key so both schemas survive.

Problem

Tools like FastAPI emit duplicate title values for input and output
variants of the same model (for example Thing-Input and Thing-Output
both carrying title: Thing). The class name is derived from the title,
so the second variant collides with the first and is rejected with
Attempted to generate duplicate models. The generator emits a warning
but exits 0, so the schema and every endpoint that referenced it are
silently dropped from the generated client.

Minimal reproduction

components:
  schemas:
    Thing-Input:
      title: Thing
      type: object
      properties:
        name: { "type": "string" }
    Thing-Output:
      title: Thing
      type: object
      properties:
        name: { "type": "string" }
        id:   { "type": "string" }

Before this PR: Thing-Output and any endpoint that references it are
absent from the generated client, and the only signal is a warning on
stderr.

Fix

In ModelProperty.build, when the title-derived class name is already
taken, fall back to a class name derived from the schema key
(Thing-Output becomes ThingOutput) provided that key is unique.
The original schema's name is preserved, and both variants and their
endpoints generate.

The fallback is guarded by data.title and name so it only triggers
in the duplicate-title case. If the fallback name also collides, the
existing Attempted to generate duplicate models error path still
fires unchanged.

Tests

• Functional test:
  end_to_end_tests/functional_tests/generated_code_execution/test_title_collisions.py
  — uses the inline spec above and asserts both Thing and
  ThingOutput exist and round-trip correctly.
• Unit test in
  tests/test_parser/test_properties/test_model_property.py
  (TestBuild::test_model_name_conflict_fallback) exercises the new
  branch in ModelProperty.build directly.

Local verification

pdm install
pdm run ruff format . --check
pdm run ruff check .
pdm mypy --show-error-codes
pdm test_with_coverage

All green on Python 3.14. 100% coverage on the changed file
(model_property.py). Golden-record snapshot tests pass without
regeneration — no generated output changes for any existing spec in
the upstream baseline.

Risk

• Ordering: the first variant encountered wins the title-derived
  name; the second falls back to the schema-key-derived name. For the
  motivating Thing-Input / Thing-Output case this is the desired
  result. A schema author who expected the second variant to claim
  the title would see different output, but that case previously
  errored out, so this is a strict improvement over the prior failure
  mode.
• No public-API change. The fix is a single conditional in
  ModelProperty.build; nothing else in the parser or templates is
  touched.

Related

Companion PR: #1449 — a second independent fix for
freeform-object defaults, surfaced from the same downstream spec.
The two PRs share no code paths and can land in either order.