fix: don't route specced Mocks through str/dict tracing

[DawnofGenX]

Problem:

Enabling hook tracing makes a successful hook call raise TypeError when an
argument or first-result return value is a specced unittest.mock.Mock.

for spec in (str, dict):
    ...
    value = Mock(spec=spec)
    pm.enable_tracing()
    pm.hook.echo(value=value)

Mock(spec=str) → TypeError: object of type 'Mock' has no len() (before the
implementation runs). Mock(spec=dict) → TypeError: 'Mock' object is not iterable (while tracing the result).

Fix:

Mock(spec=...) reports its spec through __class__, so the isinstance(obj, str)
and isinstance(args[-1], dict) checks in _tracing.py matched the mocks and
routed them into the string-formatting / dict-metadata paths. Switched both to
exact-type checks (type(obj) is str, type(args[-1]) is dict), so only real
strings and dicts take those paths and a specced Mock renders as an ordinary
value.

Verification:

• Added test_tracing_specced_mock_does_not_raise (parametrized over str and
  dict): both cases fail with TypeError without the fix, pass with it.
• Full suite: 221 passed. pre-commit run --all-files: all hooks pass.