fix(toolshed): accept // seals so C/C++ generated files can be sealed (#2539)

`check_generated_file_seals.py` declares three comment styles for the seal
line, one per generated-file family:

    _COMMENT_CHARS = {".py": b"#", ..., ".rst": b"..", ".c": b"//",
                      ".cpp": b"//", ".h": b"//"}

and `validate_generated_file_seal` compares the seal's captured prefix
against `expected_comment_prefix(filepath)` so a `.rst` file cannot be
sealed with a `#`, and so on. But the marker regex only ever accepts two of
the three:

    rb"^(?P<prefix>#|\.\.) "

`//` can never be captured, so `fullmatch` returns None for any sealed
`.c` / `.cpp` / `.h` file and it is rejected as `MALFORMED generated-file
seal` before the prefix comparison runs at all. The `b"//"` entries in
`_COMMENT_CHARS` and the branch that would validate them are dead.

Add `//` to the alternation, with a note tying it to `_COMMENT_CHARS` so
the two do not drift again.

This also adds the first tests for the script, under `toolshed/tests/`, and
runs them alongside the existing `ci/tools/tests` in the nightly tooling
job. The parametrized case is driven from `_COMMENT_CHARS` itself, so a
future entry whose prefix the regex cannot match fails immediately instead
of silently becoming dead code.

Author: LeSingh1

toolshed/check_generated_file_seals.py

@@ -16,7 +16,10 @@
 assert GENERATED_FILE_MARKER_FRAGMENT in GENERATED_FILE_SEAL_TOKEN
 _TOKEN_BYTES = GENERATED_FILE_SEAL_TOKEN.encode("ascii")
 _MARKER_REGEX = re.compile(
-    rb"^(?P<prefix>#|\.\.) "
+    # Keep the alternation in sync with the values of _COMMENT_CHARS below:
+    # a prefix that is not matched here can never reach the
+    # expected_comment_prefix() comparison in validate_generated_file_seal().
+    rb"^(?P<prefix>#|\.\.|//) "
     + re.escape(_TOKEN_BYTES)
     + rb" format=(?P<format>[0-9]+); content-sha256=(?P<digest>[0-9a-f]{64})\n$"
 )