Add --ledger to stellar keys address

[fnando]

What

Add a --ledger flag to stellar keys address that connects to an attached
Ledger hardware wallet and prints the Stellar address derived at
m/44'/148'/N'. N defaults to 0 and can be set with --hd-path N.

stellar keys address --ledger
stellar keys address --ledger --hd-path 1

The flag is scoped to keys address only. public_key::Cmd was split into
Args (still flattened by keys fund) and Cmd (used by keys address and
adds --ledger), so keys fund does not inherit the new flag. --ledger is
mutually exclusive with the positional identity name; if neither is provided,
clap rejects the command. Errors from a missing/locked device or a closed
Stellar app surface through the existing signer::ledger error chain.

Verified end-to-end against a real Ledger device.

Closes #2556.

Why

Knowing the address derived from a Ledger device is a fundamental operation —
needed before funding, before sharing an address with a counterparty, and
before scripting any workflow that involves a hardware wallet. Without
--ledger, users have to open Ledger Live or a web wallet just to look up
their own address, which breaks the CLI-first development experience.

Known limitations

• --ledger is intentionally only on keys address. keys fund --ledger
  (and other commands) will be added in separate changes.
• Nothing is persisted — there is no keys add --ledger yet, so each
  invocation re-derives from the device and --hd-path lives only on the
  current call.

[Copilot]

Pull request overview

This PR adds Ledger-backed address lookup to stellar keys address, refactors the shared key-address argument parsing, and updates related help text. In the Stellar CLI, it extends key-management flows so users can derive a hardware-wallet address directly from the CLI.

Changes:

• Split keys public-key into shared Args plus a Cmd that adds --ledger for keys address.
• Updated keys fund to reuse the shared args without exposing the new flag.
• Refreshed help/docs and removed an unused soroban-test helper.

Review findings:

• keys fund still accepts positional ledger / ledger:N, so Ledger support is not actually scoped to keys address.
• The new keys address help text/docs still imply omitting NAME falls back to a default identity, but the command now errors unless NAME or --ledger is provided.
• The new Ledger runtime path only has parser/unit coverage; it is missing soroban-test emulator/integration coverage.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
FULL_HELP_DOCS.md	Documents the new keys public-key/address Ledger option and revised usage text.
cmd/soroban-cli/src/commands/keys/public_key.rs	Adds --ledger, splits shared args, and introduces parser tests for keys address.
cmd/soroban-cli/src/commands/keys/fund.rs	Switches keys fund to the shared public-key args and adds a parser test rejecting --ledger.
cmd/crates/soroban-test/src/lib.rs	Removes an unused TestEnv::test_address helper.