fix: preserve numeric protocols in Cisco and Cisco object-group ACLs (#170)

[hardikkaurani]

Summary

Fixes #170

When a policy defines a numeric protocol (for example, protocol:: 50), the Cisco generator (cisco.py) previously rendered rules with None instead of the protocol number:

permit None any host 10.0.0.1

This occurred because cisco.Term.__str__ and cisco.ObjectGroupTerm mapped non-allowed string names via self.PROTO_MAP.get(proto). Because PROTO_MAP maps protocol names to numbers ('tcp' -> 6, 'esp' -> 50), looking up an already numeric protocol returned None.

Solution

1. In cisco.Term.__str__ and cisco.ObjectGroupTerm: use self.PROTO_MAP.get(proto, proto) as fallback so numeric protocols retain their numerical value instead of resolving to None.
2. In IOSXEMap.GetProtocolName: attempt numeric integer lookup if string lookup raises KeyError, ensuring string protocol numbers (e.g. '50') correctly map to their IOS-XE names (e.g. 'esp') when configure_replace_compatible is enabled.
3. In cisco.Term.__init__: ensure ALLOWED_PROTO_STRINGS is scoped per-instance to prevent mutating and duplicating entries in the class-level attribute across multiple term instantiations.
4. Added regression tests in tests/lib/cisco_test.py covering standard extended, object-group, IPv6, and configure_replace_compatible modes, as well as class attribute immutability.

Validation

• Ran pytest tests/lib/cisco_test.py tests/lib/ciscoxr_test.py tests/lib/ciscoasa_test.py tests/lib/arista_test.py: 91 passed.
• Ran flake8 linter with repository CI flags: 0 errors/warnings.

[hardikkaurani]

Hi @david-wu!

Could you please take a look at this PR when you have a moment? It addresses #170 where raw numeric protocols in policy definitions rendered as None in the Cisco generator. All unit tests and linters pass cleanly. Thank you!