geotiff: make _FileSource a context manager (#2449)

[brendancol]

Summary

Adds __enter__ / __exit__ to _FileSource so callers can write with _FileSource(path) as src: instead of wrapping every open in a try / finally.

Sweeps the mechanical call sites in _vrt_validation.py, _vrt.py, _backends/gpu.py, and the four test modules that exercised the helper directly. The mmap-cache refcount semantics are unchanged; __exit__ just calls the existing close().

Two production sites are left as-is. _reader._read_to_array and _backends/gpu._read_geotiff_gpu_eager both wrap a several-hundred-line function body in a try / finally and mix _FileSource with _CloudSource. Converting those would mean either teaching _CloudSource the protocol or restructuring large blocks, which is outside the mechanical scope flagged in #2449.

Test plan

• New tests xrspatial/geotiff/tests/test_file_source_context_2449.py cover __enter__ returning self, refcount release on normal exit, release on exception, double-close safety, and nested-with refcount accounting.
• pytest xrspatial/geotiff/tests/ (non-gpu / non-integration): 5096 passed, 65 skipped, 2 xfailed.
• pytest xrspatial/geotiff/tests/gpu/: 384 passed, 3 skipped.
• pytest xrspatial/geotiff/tests/integration/: 474 passed.

Closes #2449.

[brendancol]

PR Review: geotiff: make _FileSource a context manager (#2449)

Blockers

None.

Suggestions

None. The refactor is mechanical and the behaviour is preserved at every call site.

Nits

• xrspatial/geotiff/tests/test_file_source_context_2449.py:21 uses __import__('os').path.realpath(path) instead of a top-level import os. The inline import keeps the helper self-contained but is unusual; a top-level import os reads more naturally.
• xrspatial/geotiff/tests/test_file_source_context_2449.py:50 has a del src line annotated "unreachable" after a call that always raises. Dead code; can drop the line and the trailing comment.
• xrspatial/geotiff/_sources.py:332 returns False from __exit__ explicitly. That matches the implicit None return; either is fine, but dropping the return keeps the body to a single line.

What looks good

• __enter__ returns self, __exit__ calls the existing close(). Idiomatic, and the mmap-cache refcount semantics are preserved.
• _vrt_validation.py:559 flattens the nested try / except / finally into try / with / except. The outer except catches OSError both from the constructor (matching the old inner try) and from the read/parse calls, so the observable behaviour is the same with fewer indentation levels.
• _vrt.py:1958 is a latent fix in addition to the cleanup. The old loop only called src.close() on the success path, so a parse_header exception would leak a cache refcount. The with block closes on both paths.
• New tests cover __enter__ identity, refcount release on normal exit, refcount release on exception, double-close safety, and nested-with refcount accounting.
• The PR body is explicit about the two intentionally-skipped production sites (_reader._read_to_array, _backends/gpu._read_geotiff_gpu_eager). Both wrap several hundred lines and mix _FileSource with _CloudSource, so converting them is outside the mechanical scope #2449 was scoped to.

Checklist

• Algorithm matches reference/paper: N/A (helper refactor)
• All implemented backends produce consistent results: no backend-specific behaviour changes
• NaN handling is correct: N/A
• Edge cases are covered by tests: exception path, double-close, nesting all covered
• Dask chunk boundaries handled correctly: N/A
• No premature materialization or unnecessary copies
• Benchmark exists or is not needed: not needed
• README feature matrix updated (if applicable): not applicable
• Docstrings present and accurate: __enter__ / __exit__ are self-evident protocol methods; the class docstring already covers the source

[brendancol]

Follow-up review: nits addressed

Original nits, all fixed in a44d532:

• _sources.py __exit__ no longer returns False; falls through to implicit None.
• test_file_source_context_2449.py now uses a top-level import os instead of __import__('os').
• test_exit_releases_on_exception drops the unreachable del src line and the trailing # always raises comment.

Re-ran pytest xrspatial/geotiff/tests/test_file_source_context_2449.py: 5 passed.

No further findings.