Add stellar token allowance subcommand

[fnando]

What

Adds stellar token allowance, a read-only subcommand returning the SEP-41 allowance --from granted --spender. Prints the raw smallest-unit value by default, or a decimal-aware value with --decimal (which reads the token's decimals), as text or JSON.

Why

Part of #2620 (typed SEP-41 + SAC client), completing the read group. Mirrors balance, reusing args::invoke_by_position, args::not_deployed_error, and FixedPoint. Stacked on token-approve, so its tests seed state with a clean approve→allowance round-trip.

Known limitations

N/A

[Copilot]

Pull request overview

Adds the read-only stellar token allowance command, completing another SEP-41 read operation from #2620.

Changes:

• Queries raw or decimal-formatted allowances.
• Supports text, JSON, and structured errors.
• Adds help documentation and integration tests.

Reviewed changes

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

Show a summary per file

File	Description
FULL_HELP_DOCS.md	Documents the new command.
cmd/soroban-cli/src/commands/token/mod.rs	Registers and dispatches allowance.
cmd/soroban-cli/src/commands/token/allowance.rs	Implements allowance querying and formatting.
cmd/soroban-cli/src/cli.rs	Enables JSON error output.
cmd/crates/soroban-test/tests/it/integration/token/mod.rs	Registers the test module.
cmd/crates/soroban-test/tests/it/integration/token/allowance.rs	Tests values, decimals, zero allowance, and deployment errors.

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.