Make the file-type/extension provider authoritative for dynamic extensionless file associations (follow-up)

Follow-up to an earlier change that stopped tag-parsing non-included extensionless files.

Currently, dynamically discovered extensionless file associations are handled via a separate lookup outside the main file-type resolution path. Fold these into the extension/file-type provider so that the central file-type lookup is the single source of truth, rather than relying on a pre-built known list plus a separate check.

Why: As more code is unified onto the shared path, the common code won't know about these extra extensionless files unless they're part of the standard file-type resolution call. Keeping it in one place avoids divergence.

Approach options:

• Subclass/extend the extension provider and override the lookup to augment or replace the defaults, or
• Override at the host level: call the default provider first, and if it returns "none" and the file has no extension, fall back to checking the custom list.

Note: There are two consumers to keep working — the main extension and the unit tests.

Internal task number: 3021361

Locate the extension/file-type provider and the separate dynamic extensionless-association lookup, then trace their callers in the main extension and unit tests. Compare the provider and host-level approaches described in the issue; done means the central lookup handles these associations and both consumers continue to work.