Expose gRPC call timeout

[samuel-williams-shopify]

Motivation

Middleware and service handlers may need to log the client-supplied gRPC timeout. Currently they must reach into the underlying request headers and parse grpc-timeout themselves.

Changes

• add Protocol::GRPC::Call#timeout
• return the client-supplied timeout normalized to seconds, or nil when absent
• use the typed grpc-timeout header established by the gRPC header policy
• document the accessor in the unreleased notes

This keeps Call#deadline as the runtime deadline and Call#time_remaining as the current remaining budget, while Call#timeout exposes the original requested duration.

Verification

• bundle exec bake test (179 tests, 333 assertions)
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib