Add stellar token approve subcommand

[fnando]

What

Adds stellar token approve, a write subcommand that sets a SEP-41 allowance. --from grants and authorizes the allowance, --spender is the delegate, --amount is the allowance in smallest units, and --expiration-ledger is the ledger after which it expires. Returns a JSON receipt with the tx hash.

Why

Part of #2620 (typed SEP-41 + SAC client), first of the write commands after the read-metadata group. A thin wrapper over contract invoke reusing args::invoke_by_position and args::not_deployed_error, mirroring transfer. Landing approve before allowance lets the allowance command's tests do a clean approve→read round-trip.

Known limitations

Muxed (M…) source accounts are rejected with a clear error (same constraint as transfer, see #2645).

[Copilot]

Pull request overview

Adds stellar token approve for setting SEP-41 allowances and returning JSON transaction receipts.

Changes:

• Implements approval invocation, validation, signing, and structured errors.
• Registers command routing and updates help documentation.
• Adds integration coverage for approval behavior and failures.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
FULL_HELP_DOCS.md	Documents the command and options.
cmd/soroban-cli/src/commands/token/mod.rs	Registers and dispatches approve.
cmd/soroban-cli/src/commands/token/approve.rs	Implements approvals; muxed-source coverage is missing and its guard explanation is inaccurate.
cmd/soroban-cli/src/cli.rs	Enables JSON error formatting.
cmd/crates/soroban-test/tests/it/integration/token/mod.rs	Registers approval integration tests.
cmd/crates/soroban-test/tests/it/integration/token/approve.rs	Tests approval behavior and key failures.

Suppressed comments (1)

cmd/soroban-cli/src/commands/token/approve.rs:158

• This explanation contradicts #2645: positional parsing and simulation accept the muxed address; the failure occurs later when the sequence-number lookup passes the M… strkey to a G-only account parser. Please keep the guard but describe the actual failure point so future work on #2645 does not remove or relocate it for the wrong reason.

        // The invoke pipeline can't source a transaction from a muxed account
        // yet (see #2645), and a muxed strkey in the `from` arg is rejected
        // mid-simulation with an opaque host error; reject it up front with a
        // clear message instead.

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.

[leighmcculloch]

The CLI validates --amount is non-negative (via parse_nonneg_i128, see the Error::Args/args::Error layer) to avoid an opaque HostError, but this doc comment states the same trap condition applies to --expiration-ledger — a positive amount with an expiration ledger already in the past — yet there's no equivalent validation.

[leighmcculloch]

Fwiw receipt is not a common term in the Stellar vocab. I'd call this Result rather than introduce a new concept. It's not a blocker though, as it appears to just be a term being used internal to the cli.