chore(release): prepare 0.2.0

[moonyue-w]

Summary

Prepare the Python SDK for 0.2.0: update pyproject.toml and the root package version in uv.lock, move the reviewed changes into the dated 0.2.0 changelog entry, and refresh release documentation. Keep an empty Unreleased section and preserve all breaking-change migration guidance and 0.1.0 history.

Why

The release workflow requires the package version and a nonempty changelog entry to match the requested release. The minor-version bump follows the pre-1.0 compatibility policy for the response-validation and error-classification changes already merged into main.

Verification

• uv lock --check; dependency versions and markers remain unchanged.
• uv run make lint
• uv run make typecheck
• uv run make test: 2,994 passed; 18 account-backed integration cases deselected as configured.
• uv run make docs-check: passed; 25 README Python snippets compiled.
• Built wheel and sdist; confirmed wheel metadata reports 0.2.0.
• Changelog validation and extraction for 0.2.0; 19 release-notes automation tests passed.
• Example --help checks were not rerun; example source is unchanged.
• Account-backed integration tests are not required for this version/documentation-only PR; the source baseline passed integration before the version bump.

Impact

• Installed package metadata supplies qca.__version__; no separate runtime version constant needs updating.
• Reviewed user-facing notes moved from Unreleased to 0.2.0 without alteration, including strict-validation opt-in and HTTP 529 migration guidance.
• Synchronous/asynchronous APIs, generated API documentation, and contract fixtures are unchanged; documentation checks passed.
• Go and TypeScript release versions are coordinated at 0.2.0.
• No credentials, environment files, logs, or generated test output committed.