Include .t files in Perl::LanguageServer fileFilter

Serena's file-to-language router already maps `.t` to the Perl LS
(`Language.PERL` in `solidlsp/ls_config.py`), but the
Perl::LanguageServer-side `fileFilter` was hardcoded to
`[".pm", ".pl"]`. As a result Perl::LanguageServer skipped `.t`
files entirely: symbol queries on a `.t` returned empty, and
references to `.pm`/`.pl` subs missed any callers defined in
`.t` files.

Add `.t` to the filter, deduplicate the two `fileFilter` literals
into a `_FILE_FILTER` class constant, and note the sync requirement
with `Language.PERL.get_source_fn_matcher()` in a comment. Cover
the behavior with a new cross-file references test plus a `helper.t`
fixture; the test fails on unpatched `main` and passes on this
branch.

test/solidlsp/perl/test_perl_basic.py

@@ -77,6 +77,14 @@ class TestPerlLanguageServer:
         assert 17 in main_pl_lines, f"Expected reference at line 18 (0-indexed 17), found: {main_pl_lines}"
         assert 20 in main_pl_lines, f"Expected reference at line 21 (0-indexed 20), found: {main_pl_lines}"
 
+    @pytest.mark.parametrize("language_server", [Language.PERL], indirect=True)
+    def test_find_references_includes_t_files(self, language_server: SolidLanguageServer) -> None:
+        """References to a .pm/.pl sub must surface callers in .t test files (fileFilter includes .t)."""
+        reference_locations = language_server.request_references("helper.pl", 4, 5)
+
+        t_refs = [ref for ref in reference_locations if ref["uri"].endswith(".t")]
+        assert t_refs, f"Expected at least one reference in a .t file, got: {[r['uri'] for r in reference_locations]}"
+
     @pytest.mark.parametrize("language_server", [Language.PERL], indirect=True)
     def test_bare_symbol_names(self, language_server) -> None:
         all_symbols = request_all_symbols(language_server)