BUG: Read the extension from the final path component in get_ext

[hjmjohnson]

get_ext() scans the entire path for ., so a dot in any parent directory is mistaken for the file extension. For a version-numbered build/output path like .../build-5.4/…/trx_test (a file with no extension), get_ext returns "4/…/trx_test", which the writer rejects with "Unsupported extension". This fixes it to read the extension from the final path component only.

Root cause

// before
const std::size_t pos = str.rfind('.');   // '.' anywhere in the path

rfind('.') on the full path finds the dot in build-5.4, not in the filename:

get_ext(".../build-5.4/out/trx_test") = "4/out/trx_test"   -> "Unsupported extension"
get_ext(".../build-main/out/trx_test") = ""                -> ok (no dot in path)

The fix restricts the search to the substring after the last / or \, then takes the part after that component's last ..

No change to dtype extraction

get_ext is also used to read the dtype from array element filenames (positions.3.float32, offsets.uint64). Those carry the dot in the basename, so the returned dtype is unchanged — the fix only differs when the basename has no dot but a parent directory does.

Testing

Adds TrxFileIo.get_ext_uses_final_path_component:

get_ext("/a/build-5.4/out/trx_test")        == ""          // the bug case
get_ext("/a/build-5.4/out/trx_test.trx")    == "trx"
get_ext("out.trx")                          == "trx"
get_ext("trx_test")                         == ""
get_ext("/a/shard.1/positions.3.float32")   == "float32"    // dtype filename, dotted parent
get_ext("offsets.uint64")                   == "uint64"
get_ext("/a/b.c/name.")                     == ""           // trailing dot

Full ctest suite passes 185/185 (the dtype callers are exercised by test_io).

Found via an ITK v5-vs-v6 downstream build testbed whose build path contained a dotted directory; the three ITKTractographyTRX tests that failed there pass with this change.

[hjmjohnson]

@mattcieslak Some fixes exposed while trying to build TRX with both ITKv5 and ITKv6.