feat: Add streaming API support and documentation

[mad4ms]

Introduce streaming support and a more predictable REST client architecture for the Sportradar DataCore API.

Streaming API

• streaming.py provides HandballStreamClient and HandballStreamingAPI for MQTT access grants and fixture or venue streams.
• stream_models.py defines typed stream grants, messages, and publish results.
• stream_utils.py handles topic and payload decoding, including compressed payloads.
• The stream extra keeps paho-mqtt optional and imports it lazily.

REST configuration and lifecycle

• REST authentication is lazy: constructing DataCoreAPI performs no network request.
• connect() remains available for explicit initialization, while the first endpoint call authenticates automatically.
• DataCoreSettings provides typed configuration and HandballAPI.from_env() supports environment variables plus optional .env loading.
• Existing explicit constructor configuration remains supported.

Generated client and codegen

• The generated OpenAPI client now has one package location: src/datacore_client/.
• The _vendor import indirection was removed to improve packaging and IDE/type-checker resolution.
• Bash and PowerShell codegen use the same prepare_openapi_spec.py preprocessing path.
• The prepared spec is validated before generation, and openapi-python-client==0.26.1 is pinned for reproducibility.

Documentation and tests

• README and architecture, development, API, and error documentation were updated.
• Agent instructions document generated-client boundaries and the supported configuration flow.
• Focused configuration tests were added.

Verification

• pytest test -q: 14 passed, 1 skipped because STREAM_FIXTURE_ID is not set.
• Ruff: passed.
• Mypy: passed.
• OpenAPI preprocessor smoke test: passed.

The generated client remains build output and should only be changed through the codegen scripts.