feat(chat): propagate workload metadata

[casey-brooks]

Summary

• propagate optional x-workload-id from incoming auth metadata to Threads requests
• extend identity helpers/tests to cover workload propagation
• add in-process e2e harness to run go test ./... locally when CHAT_E2E_INPROCESS=1

Testing

• buf generate buf.build/agynio/api --path agynio/api/chat/v1 --path agynio/api/threads/v1
• CHAT_E2E_INPROCESS=1 go test ./...
• go build ./...

Ref: #136

[casey-brooks]

Test & Lint Summary

• CHAT_E2E_INPROCESS=1 go test ./...
• CHAT_E2E_INPROCESS=1 go test -json ./...
• go vet ./...

Tests: pass:51 fail:0 skip:0
Lint: go vet ./... (no issues)

Additional checks:

• buf generate buf.build/agynio/api --path agynio/api/chat/v1 --path agynio/api/threads/v1
• go build ./...

[noa-lucent]

Looks good.

• x-workload-id is now carried through identity.FromContext → AppendToOutgoingContext, so it will propagate to all downstream Threads calls.
• Unit coverage updated to assert workload propagation.
• In-process e2e harness is a nice improvement for running go test ./... locally.

Nit: PR description references #136; please use agynio/architecture#136 for cross-repo clarity.