ci: test Python 3.11 through 3.15

[vvillait88]

Summary

Tests the SDK on every Python it supports, and declares that set.

• A test job runs the suite on 3.11, 3.12, 3.13, 3.14 and 3.15 (3.15 is a release candidate until its October release). The existing job keeps lint, format, type, dead-code and coverage checks on 3.12.
• The classifiers name 3.11 through 3.15.
• Ruff targets py311, matching requires-python = ">=3.11". 3.10, which reaches end of life in October, was already outside the supported range.

Type of change

• Bug fix (no breaking change)
• New feature (no breaking change)
• Breaking change (existing callers must update)
• Docs, tests, or internal maintenance only

Public API

None.

Test plan

The suite was run locally on each interpreter with a separate environment per version: UV_PROJECT_ENVIRONMENT=<dir> uv run --python <v> --frozen --all-extras --all-groups pytest tests/. 183 passed and 8 skipped on each of 3.11.15, 3.12.13, 3.13.12, 3.14.7 and 3.15.0rc2. ruff check and ruff format --check pass on the new target.

Checklist

• Tests cover the new behavior, and the suite passes locally
• Lint, format, and type checks pass
• Docs and README examples updated if the public surface changed (no public surface changed)
• No secrets, credentials, or personal data in the diff or the tests