[Nexthop][fboss2] Show FEC histogram for all interfaces when none specified

[zackary-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

fboss2 show interface counters fec <line|system> histogram returned an empty table when no interface was specified. It didn't enumerate ports at all: it just returned an empty table (if (queriedIfs.empty()) return RetType{};). Now enumerates via qsfp_service's getPortMediaInterface, which reports exactly the ports with a transceiver physically present. Ports that fail the per-interface CDB lookup are skipped silently in the enumerate-all case, or reported to stderr when explicitly requested.

Test Plan

• Added unit tests covering explicit interface, fallback to all interfaces, and skip-on-throw. bazel test //fboss/cli/fboss2/test:cmd_test --test_filter="*FecHistogram*" — 3/3 passed.
• Verified on a switch: command now lists all up interfaces with transceivers instead of an empty table.

[meta-codesync]

@srikrishnagopu has imported this pull request. If you are a Meta employee, you can view this in D113635441.