feat: support --account and explicit credentials for get-context-token

[lewiszlw]

This PR modifies the get-context-token subcommand to support two authentication modes, aligned with the existing send subcommand:

1. Saved Account Mode: --account <index>
2. Explicit Credentials Mode: --bot-token <token> --user-id <user_id> [--route-tag <route_tag>]

When no authentication parameters are provided, the command falls back to the first saved account (preserving backward compatibility).

Changes

• src/cli.rs: Expanded GetContextTokenArgs to include account, user_id, bot_token, and route_tag fields.
• src/commands/send.rs: Made SendTarget and resolve_send_target public so they can be reused.
• src/commands/get_context_token.rs: Reused SendTarget and resolve_send_target from send.rs, removing the previously duplicated Target enum and resolve_target function.
• src/main.rs: Updated the GetContextToken command dispatch to pass the new arguments.

Validation

• cargo build — success
• cargo test — 15 passed; 0 failed
• cargo clippy -- -D warnings — success (no warnings)