fix: allow Rust consumers to link datafusion-python

[emecii]

Which issue does this PR close?

Closes #1703.

Rationale for this change

Unconditionally enabling pyo3/extension-module prevents ordinary Rust executables from linking the core rlib against libpython. It also prevents CI from executing useful Rust tests.

What changes are included in this PR?

• Expose a default-on extension-module crate feature and gate the extension-only build-script linker arguments with it.
• Explicitly enable that feature for all wheel builds, including free-threaded builds that disable defaults to avoid ABI3.
• Add a separate integration-test executable that constructs PySessionContext from Rust, initializes Python, executes SQL through the bindings, and asserts a result. Run core Rust tests in Linux CI with defaults disabled and Substrait enabled.
• Document the test/dependency configuration and the workspace feature-unification caveat.

Are there any user-facing changes?

Rust executables can depend on datafusion-python with default-features = false and opt into features such as substrait. Default Python extension builds retain ABI3 and mimalloc; Python APIs are unchanged.

Validation

Local macOS arm64 validation:

• The new rust_link executable fails before the fix with unresolved Py_* symbols and passes after it. Both the minimal no-default-feature command and cargo test --locked -p datafusion-python --no-default-features --features substrait execute and pass the test.
• All-target Cargo checks with defaults and without defaults; no-default-feature Clippy with -D warnings; all-target/all-feature Clippy through pre-commit.
• Built and installed the ABI3 wheel with maturin 1.13.3. SQL/Arrow smoke check and the complete Python tests/doctests pass: 1,387 passed, 7 skipped.
• Built and installed the CPython 3.14t wheel with defaults disabled and extension-module,mimalloc,substrait enabled. SQL/Arrow smoke check passes and the GIL remains disabled after import and execution.
• The composite action's actual argument script passes 54 combinations plus GIL-interpreter rejection in its Windows branch (branch logic exercised locally, not a Windows-host build).
• Nightly rustfmt, Taplo, native Actionlint 1.7.12, and the remaining full pre-commit hooks pass. Docker is not installed locally, so the Docker Actionlint hook was replaced by the same pinned native version.

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