[countersyncd]: fix 202412 backport cleanup regressions

[Pterosaur]

What I did

Fix follow-up cleanup regressions in the recent countersyncd 202412 backports.

Specifically:

• remove stale object_names_map updates left behind after the object_id_name_map conversion in ipfix.rs
• update ipfix.rs tests to use the current IPFixTemplatesMessage::new(..., object_names, object_ids) signature
• update the stale-mapping test to validate object_id_name_map
• mark legacy SwssActor wrapper helpers as #[cfg(test)] so non-test builds do not fail under RUSTFLAGS=-Dwarnings

Why I did it

The recent 202412 backport stack for sonic-net/sonic-swss PRs 4316/4317/4318/4319 had two cleanup gaps:

1. #4318 backport (41c6d27b) renamed the actor field to object_id_name_map but left stale object_names_map references in ipfix.rs and related tests.
2. #4319 backport (4150bafc) introduced static helper paths in swss.rs while leaving older instance wrapper methods only used by tests, which trips dead_code when building with RUSTFLAGS=-Dwarnings.

This PR makes the 202412 branch compile and test cleanly again.

How I verified it

In a CI-like sonic-slave-bookworm container:

RUSTFLAGS=-Dwarnings cargo test --locked --manifest-path crates/countersyncd/Cargo.toml

Result:

• unit/integration/doc tests passed
• exit code 0

Details if related

This is a fix-up for the recent 202412 backports corresponding to:

• [countersyncd]: resolve object names by template id sonic-net/sonic-swss#4316
• [countersyncd]: align multi-message netlink parsing sonic-net/sonic-swss#4317
• [countersyncd]: resolve labels via object ids sonic-net/sonic-swss#4318
• [countersyncd]: avoid blocking async actor loops sonic-net/sonic-swss#4319