Add Python 3.15 support

[jonathan343]

Summary

Python 3.15.0 is out, but the runtime packages and generated clients still list 3.14 as the newest supported version, and the 3.15 CI job added in #688 is non-blocking. This PR adds the Programming Language :: Python :: 3.15 classifier to every package under packages/ and to the pyproject.toml that SetupGenerator writes for generated clients. It regenerates the codegen-output/ snapshots and makes the 3.15 CI job blocking.

It also fixes test_aws_json11_raises_parse_error_for_invalid_error_body, which only passed when ijson used its C backend. The locked ijson 3.3.0 ships wheels only up to cp312, and no ijson release ships a cp315 wheel yet. When ijson is installed without its C extension, it falls back to the pure-Python backend. That backend raises IncompleteJSONError with Incomplete JSON content, which the test's premature EOF|parse error pattern rejected. The pattern now accepts all three messages.

Upgrading the locked ijson would add wheels for 3.13 and 3.14 but not 3.15, so it is left for a separate PR.

Before / After

Run the test with ijson forced to its pure-Python backend:

IJSON_BACKEND=python uv run pytest \
  "packages/smithy-aws-core/tests/unit/aio/test_protocols.py::test_aws_json11_raises_parse_error_for_invalid_error_body" \
  --no-cov -q

Before (develop)

E       AssertionError: Regex pattern did not match.
E         Expected regex: 'premature EOF|parse error'
E         Actual message: 'Incomplete JSON content'

packages/smithy-aws-core/tests/unit/aio/test_protocols.py:470: AssertionError
1 failed in 0.28s

After

1 passed in 0.08s

Testing

The full runtime suite also passes on the pure-Python backend, so no other test depends on the C backend:

IJSON_BACKEND=python uv run pytest packages --no-cov -q

2404 passed, 9 skipped, 1 warning in 19.55s

This ran locally on macOS with Python 3.12. CI covers the C backend, which the old pattern already matched.

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