feat(config): avoid duplicate pair_reg event on re-registration

[Baskarayelu]

Summary

register_pair/register_pairs are documented as idempotent, but previously fired a fresh pair_reg event on every call, including a redundant re-assertion of an already-registered pair. This adds a check against the existing registration state so pair_reg only fires on the genuine unregistered→registered transition, satisfying the 'no duplicate emissions' requirement for config-state-change events.

Adds two tests: single register_pair re-call doesn't duplicate the event, and a register_pairs batch mixing an already-registered pair with a fresh one only emits for the fresh one.

Test output

As noted in #409's PR, main currently fails cargo build --tests at HEAD with 103 pre-existing, unrelated errors (duplicate symbol definitions from a prior merge, reproducible via git stash with zero local changes). This PR adds zero additional errors — confirmed by diffing the error output before/after this change (identical error set, none referencing the touched functions or new tests).

Closes #410