cli: show exchange code and signing pubkey in geolocation probe list

[ben-dz]

Summary of Changes

• doublezero geolocation probe list now displays the exchange's code (e.g. ams) instead of the exchange pubkey, falling back to the pubkey string when the exchange can't be resolved.
• Added a signing_pubkey column (sourced from the probe's metrics_publisher_pk) so users can see which key the geoprobe is signing with probe get per row. Allows a user to fill out a target's CLI arguments from only probe list, rather than having to subsequently run a probe get.
• Added list_exchanges() to the GeoCliCommand trait so the geolocation CLI can pull exchanges via the serviceability client and map pubkey → code in a single pass.
• Behavior change for JSON consumers: the exchange field is now the code string instead of the pubkey string.

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	2	+104 / -6	+98
Docs	1	+1 / 0	+1

Production change is ~17 lines; the bulk is updated tests covering the new output and the unknown-exchange fallback.

Key files (click to expand)

• smartcontract/cli/src/geolocation/probe/list.rs — swap exchange: Pubkey for resolved exchange: String + new signing_pubkey column; fetch exchanges in execute; tests updated and new fallback test added
• smartcontract/cli/src/geoclicommand.rs — add list_exchanges() to the GeoCliCommand trait and implement it on GeoCliCommandImpl using the serviceability client
• CHANGELOG.md — entry for the new column / display change

Testing Verification

• cargo test -p doublezero_cli --lib — 329 tests pass, including 4 in geolocation::probe::list (one new test, test_cli_geo_probe_list_table_unknown_exchange_falls_back_to_pubkey, covers the pubkey fallback when the exchange map is missing the entry).
• Manually checked the doublezero geolocation probe list behavior.