Add stellar token decimals subcommand

[fnando]

What

Adds stellar token decimals, a read-only subcommand returning a token's SEP-41 decimals() metadata as a u32. Resolves a token by --id (contract id, alias, native, or CODE:ISSUER) and prints the value as text or JSON.

Why

Part of #2620 (typed SEP-41 + SAC client), completing the read-metadata group after name and symbol. Stacked on the token-symbol branch; a thin wrapper over contract invoke reusing args::invoke_by_position and args::not_deployed_error.

Known limitations

N/A

[Copilot]

Pull request overview

Adds stellar token decimals for querying SEP-41 decimal metadata from tokens and SACs.

Changes:

• Implements text and JSON decimal output.
• Integrates command routing and structured errors.
• Adds help documentation and integration coverage.

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 subcommand.
cmd/soroban-cli/src/commands/token/mod.rs	Registers and dispatches decimals.
cmd/soroban-cli/src/commands/token/decimals.rs	Implements decimal metadata queries.
cmd/soroban-cli/src/cli.rs	Enables structured JSON errors.
cmd/crates/soroban-test/tests/it/integration/token/mod.rs	Registers integration tests.
cmd/crates/soroban-test/tests/it/integration/token/decimals.rs	Tests native, issued-asset, and error cases.

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