Custom-registered handlers bypass CSVW metadata discovery

[Iwan-Dyke]

Problem:
When _process_delimited_file is registered as a custom handler via register_handler, it gets called with the 3-arg signature (file_path, spark, options) instead of the 4-arg signature (file_path, spark, options, metadata_path). This silently skips CSVW metadata discovery.

# file_processor.py — the dispatch logic:
if handler == _process_delimited_file:
    result = handler(file_path, spark, options, metadata_path)  # ← 4 args, CSVW works
else:
    result = handler(file_path, spark, options)  # ← 3 args, CSVW skipped

If a consumer registers the built-in delimited handler for a new extension:

from file_processing.delimited_files import _process_delimited_file
register_handler(".myext", _process_delimited_file)

The identity check handler == _process_delimited_file fails because get_handler returns the function from the registry, but the comparison is against the module-level reference — and the registered handler goes through the custom path (3-arg call), losing CSVW support.

Expected behaviour:
Registering _process_delimited_file (or any delimited handler) for a custom extension should not lose CSVW metadata discovery.

Acceptance criteria:

• [] Custom-registered handlers that support CSVW receive metadata_path
• [] No silent loss of functionality when re-registering built-in handlers for new extensions
• [] Existing custom handlers with 3-arg signatures continue to work