feat(icrc-ledger-types): add ICRC-124 block schema validators

[bogwar]

Summary

• Add ICRC-124 block schema validators (validate_pause, validate_unpause, validate_deactivate) and ICRC-154 method name constants to icrc-ledger-types
• Block schema: btype (required), ts (required), phash (optional 32-byte blob), tx map with all-optional provenance fields (mthd, caller, reason, ts)
• First PR in a stacked series implementing ICRC-124/154 (pause/unpause/deactivate)

Test plan

• Schema validators accept minimal blocks (empty tx) and full blocks (all provenance fields populated) for each of the 3 block types
• Validators reject wrong btype, missing required fields (btype, ts, tx), invalid phash length (16 bytes instead of 32), and non-map inputs

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds initial ICRC-124/154 support to icrc-ledger-types by introducing schema validators for ICRC-124 management blocks (pause/unpause/deactivate) and exposing a new icrc124 module.

Changes:

• Add new icrc124 module export from the crate root.
• Implement shared ICRC-124 block/tx schema predicates plus validate_pause, validate_unpause, and validate_deactivate.
• Add unit tests covering valid minimal/full blocks and several invalid cases (wrong btype, missing required fields, invalid phash, non-map inputs).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/icrc-ledger-types/src/lib.rs	Exposes the new icrc124 module publicly from the crate root.
packages/icrc-ledger-types/src/icrc124/mod.rs	Declares the schema submodule for ICRC-124.
packages/icrc-ledger-types/src/icrc124/schema.rs	Implements ICRC-124 block schema validators and corresponding unit tests; defines ICRC-124 block type and ICRC-154 method name constants.

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