feat(sandbox): add an optional compatible Driver SDK extra

[f-trycua]

Sandbox's typed Fleet connection needs the compatible generated Driver SDK. This adds cua-sandbox[driver] with cua-driver==0.25.0, the public-registry lock entries, a packaging regression test, and a short install example. Base dependencies and the Release Please-managed Sandbox version (0.4.3) remain unchanged.

Refs #2512.

Validation on c9cbf88e2c21c33408c409fd6316f26053eeaf2c (Python 3.12, macOS arm64):

• Built a wheel and sdist; strict Twine checks pass.
• Fresh base-wheel install imports from the isolated environment without Driver installed: 45 packaging and typed-connection tests pass.
• Fresh extra install uses public cua-driver==0.25.0, not a locally built Driver candidate. Both modules import from installed wheels. With CUA_SANDBOX_REQUIRE_NATIVE_DRIVER=1, isolated Python and pytest importlib mode: 238 typed-connection, native bridge, pool, transport, lifecycle, and cleanup tests pass, with no skips.
• uv lock adds only Driver and its optional dependency metadata; all five wheel hashes match the exact Driver release checksums. uv lock --check passes.
• Ruff lint/format, Black, isort, dependency consistency checks, and git diff --check pass.

The Driver release and its five-platform wheel build/install gates passed in SDK publication. The local Sandbox/native bridge results above qualify the installed macOS package and simulated transport, not a live Fleet guest or Windows/Linux Sandbox extra execution.

Fresh PR CI and independent review remain the merge gates. Once merged, this feeds the Sandbox release PR (#3676); Release Please owns the version. The optional package does not deploy a Driver service or enable a hosted endpoint. Live hosted use still requires a compatible service and separately verified routing/isolation. Existing computer-server paths remain unchanged.

[codecov-commenter]

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