feat(asap-precompute-go): HttpPollChannel + OpAmpChannel (Phase 2 step 2.10)

[zzylol]

Summary

Lands the two concrete ControlChannel implementations introduced as a trait in #219, completing Phase 2 step 2.10 of the precompute-runtime extraction. The interface itself only earns its keep when at least one real impl exists, so HTTP-poll and the OpAMP stub land together.

• HttpPollChannel — pull-based, controller-driven channel: ETag-aware GET against a configurable URL, JSON body decoded into precompute.PrecomputeConfigSet, optional POST to an AckURL for plan-version receipts. Bearer tokens are read from disk on every request so credential rotation does not require a runtime restart. Errors are stashed on LastErr() (the trait is errorless by design — adapters should never crash a runtime over a transient control-plane hiccup).
• OpAmpChannel — interface-satisfying stub. Validates ServerEndpoint at construction, logs a once-per-instance warn from Poll, debug-logs each Ack, returns nil from Close. Real OpAMP wiring (github.com/open-telemetry/opamp-go/client) is deferred to Phase 5 per ADR-0003; landing it now would force a transitive dep into Phase 2 for no functional gain.

Wire format

The HTTP body is the standard encoding/json marshalling of precompute.PrecomputeConfigSet. No json tags exist on the struct yet, so field names follow Go-export casing (Version, Configs, AggID, …). Plan freshness is tracked via the HTTP ETag header: the first 200 stores the tag; subsequent polls send it back as If-None-Match and treat 304 as no-change. Ack POSTs {\"plan_version\": N} to AckURL (no-op when empty). Documented in the http_poll.go package comment so future schema work has a stable starting point.

Test plan

• go build ./... clean
• go vet ./... clean
• go test ./controlchannel/... — all green
• go test -race ./controlchannel/... — race detector clean
• httptest-driven cases:
  • first poll returns plan + stores ETag; second poll with same ETag receives 304 → returns nil
  • server rotates ETag → poll returns the new plan
  • 5xx response → Poll returns nil, LastErr reports status
  • decode failure path covered
  • bearer-token rotation: file rewritten between polls, server-side handler asserts both tokens were observed
  • missing token file → poll fails closed (server is never reached)
  • context cancellation: in-flight poll aborted by Close on another goroutine
  • AckURL POSTs {\"plan_version\": N}; empty AckURL is a no-op
  • 5xx on Ack records error without panicking
  • Close is idempotent; subsequent Poll/Ack record ErrChannelClosed
  • 8-goroutine concurrent Poll/Ack racing against Close (race-detector hardening)
• OpAmp tests: empty endpoint rejected; Poll returns nil; once-per-instance warn does not repeat; Ack/Close/idempotent-Close no-ops; trait satisfaction check

Notes

• ControlChannel interface is unchanged. Both impls are additive.
• No new external imports — stdlib only on the production path; testify is intentionally avoided to match the existing module's stdlib-testing style.
• Submodule pointer drift on opentelemetry-collector / opentelemetry-go is intentionally not committed.

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