Return None for signed values with a non-decimal timestamp

[eeshsaxena]

Fixes #3701

Problem

decode_signed_value raises ValueError instead of returning None when a v2 signed value has a syntactically malformed (non-decimal) timestamp field, even though the HMAC signature verified. _decode_signed_value_v2 does:

timestamp = int(timestamp_bytes)

which throws ValueError: invalid literal for int() with base 10: b'a'.

RequestHandler.get_signed_cookie documents that an invalid signed value returns None, so a malformed cookie should be rejected, not raise into request handling.

This is not a signature-bypass: producing such an input requires the signing key. But a malformed value from a key-holding component (or during a key rotation/migration) turns into an unhandled exception rather than a clean rejection.

Fix

Wrap the int() conversion and return None on ValueError, consistent with the other rejection paths in the same function.

Test

Added SignedValueTest.test_malformed_timestamp, which builds a correctly signed v2 value whose timestamp field is b"a" and asserts it decodes to None. It raises the reported ValueError without this change and passes with it; the full SignedValueTest suite passes (11 tests).