Move timeout formatting to header

[samuel-williams-shopify]

Motivation

Timeout serialization belongs with the typed grpc-timeout header rather than the general Protocol::GRPC::Methods utility namespace.

Changes

• add Protocol::GRPC::Header::Timeout.format for encoding durations
• keep parsing on Protocol::GRPC::Header::Timeout#to_seconds
• make numeric header coercion use Timeout.format directly
• retain Methods.format_timeout and Methods.parse_timeout as deprecated compatibility shims
• remove the dependency from Header::Timeout back to Methods
• add focused timeout-header tests and an unreleased note

Deprecated methods emit warnings only when $VERBOSE is enabled, following the existing project convention.

Verification

• bundle exec bake test (192 tests, 346 assertions)
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib