Support callable ingress_headers for per-request context propagation

[yshaul]

Summary

Allow Config#ingress_headers and #admin_headers to accept a callable (Proc, Lambda, or any object responding to #call) in addition to a static Hash. The callable is evaluated each time Restate.client is built.

Motivation

Frameworks like Rails need to inject per-request headers (team context, shard routing, auth tokens) that vary by caller context. A static Hash configured at boot time can't capture this. A callable evaluated at Restate.client call time solves this cleanly:

Restate.configure do |c|
  c.ingress_url = "http://restate:8080"
  c.ingress_headers = -> {
    { "X-Team-Id" => Current.team_id, "X-Shard" => Current.shard }
  }
end

# Later, in a request context:
Restate.client.service(MyService).do_work(args)
# => headers are resolved fresh with the current request's team/shard

Changes

• lib/restate.rb — Restate.client now calls resolve_headers which invokes .call on callable values, or passes Hashes through unchanged.
• lib/restate/config.rb — Updated RDoc on ingress_headers and admin_headers to document the callable option with examples.
• spec/callable_headers_spec.rb — 8 new tests covering: static Hash passthrough, lambda/Proc evaluation per client call, fresh evaluation on each call, mixed static+callable, and arbitrary #call responders.
• spec/introspection_spec.rb — Updated the test-local Restate.client stub to use the same resolution logic.

Backward Compatibility

Fully backward compatible — existing static Hash configs continue to work unchanged. The respond_to?(:call) check only activates for callable values.

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