Remove Fortran parser special-case for C declarations and generalize invalid-syntax handling - #36

Motivation

• Remove fragile, C-specific regex checks that attempted to recognize foreign C declarations in Fortran sources and instead treat unknown top-level lines as generic invalid Fortran syntax.
• Make the parser's top-level validation language-agnostic so non-Fortran constructs are rejected by the generic invalid-syntax path rather than by a dedicated C-declaration detector.
• Simplify the parser code by removing duplicate checks scattered in several spec/contains parsing helpers.

Description

• Deleted the _FOREIGN_C_DECLARATION and _FOREIGN_C_ALIAS_DECLARATION regexes and the _raise_if_foreign_c_syntax_line helper, removing the C-specific detection logic.
• Updated _helper_validate_file_scope_unparsed_lines to use a generic invalid-syntax diagnostic and added a clarifying docstring about the language-agnostic guard.
• Removed calls to the removed _raise_if_foreign_c_syntax_line check from specification visitors and derived-type parsing helpers so non-Fortran lines are handled by _raise_invalid_fortran_syntax_line instead.
• Adjusted tests to reflect the new generic error reporting: updated expectations in tests/parser/test_error_handling.py, tests/parser/test_cli.py, and tests/parser/c/test_c_functions.py to accept the generic invalid-syntax codes/messages.

Testing

• Ran the parser-focused test subset with pytest tests/parser which includes the modified tests, and all tests in that subset passed.
• Executed the updated tests tests/parser/test_error_handling.py, tests/parser/test_cli.py, and tests/parser/c/test_c_functions.py and they succeeded with the new generic error behavior.
• No remaining tests relied on the removed C-specific diagnostics after updating their assertions, and the parser tests completed successfully.

Codex Task