geotiff: make _FileSource a context manager (cleanup follow-up to #2444)

[brendancol]

Reason or Problem

xrspatial.geotiff._sources._FileSource exposes close() but no __enter__ / __exit__. Every call site has to wrap it in try / finally to guarantee mmap release. PR #2448 added another such call site in _vrt_validation._check_mixed_source_crs, and a code-review nit flagged the resulting try / except / finally / continue shape as harder to scan than a with block.

Proposal

Add __enter__ / __exit__ to _FileSource so callers can write:

with _FileSource(path) as src:
    data = src.read_all()
    ...

Then sweep the existing call sites (search for _FileSource( in xrspatial/geotiff/) and replace each try / finally with a with block. The mmap-cache release semantics stay the same; the helper change is mechanical.

Drawbacks

Mechanical sweep across several files. Worth verifying that no call site relies on the handle outliving a function scope before swapping wholesale.

Additional Notes or Context

Surfaced during review of PR #2448. Not blocking that PR; tracked here so the cleanup is not forgotten.