This PR:

- prepares `composio==2.0.0b0` with `composio-client==2.0.0rc6`
- keeps every Python provider package on its stable `0.x` release train
- adds a core-only build target so Python prereleases publish no
provider artifacts
- preserves the existing SDK-and-provider build path for stable Python
releases
- updates the release guard and changelog for independently versioned
SDK prereleases

Verification:

- `uv lock --check`
- `pnpm test:release-workflow`
- `go run github.com/rhysd/actionlint/cmd/actionlint@v1.7.10
.github/workflows/py.release.yml`
- `make build-core` and `twine check --strict dist/*`
- installed the built wheel with all 12 published provider packages at
`0.21.0`; `uv pip check` and all provider imports passed

After merge, pushing the immutable `py@2.0.0b0` tag will publish only
the `composio` wheel and source distribution.