Enforce Avro on low-level PHP outbound payload paths

[rmcdaniel]

Problem

PHP SDK 2.0.0-rc.30 rejects a non-Avro codec passed to the Client constructor, but public low-level send surfaces still bypass that boundary. Client::updateSchedule() forwards caller-supplied schedule changes unchanged, including a schedule action input envelope tagged codec: "json". Client::completeWorkflowTask() likewise forwards caller-supplied command payload envelopes unchanged. Both requests reach the transport instead of failing locally with unsupported_payload_codec.

This contradicts the public Avro-only contract and the release claim that non-Avro configuration fails before transmission. Server-side rejection remains necessary, but it does not make the SDK boundary complete.

Acceptance criteria

• Enforce the fixed Avro Value schema and single-object framing at every public PHP Client send path that can accept a caller-supplied durable payload or payload envelope.
• Cover schedule-update action inputs and low-level workflow-task completion command payloads, including JSON-tagged, unknown-codec, untagged/raw JSON, and falsely Avro-tagged non-Avro bytes.
• Reject unsupported payloads with the stable actionable unsupported_payload_codec diagnostic before Transport::send() is invoked.
• Validate only schema-defined payload locations. Memo, search attributes, failure details, and other user-owned metadata containing keys such as codec, payload_codec, or blob must not be mistaken for payload envelopes.
• Preserve valid Avro schedule updates and workflow commands, and preserve the managed Worker path that already emits Avro.
• Add focused transport-spy regressions proving rejected inputs produce zero requests, valid Avro inputs are sent, and codec-looking user metadata is preserved.

Complete when

Every public PHP SDK path that can send a durable payload either creates it with AvroPayloadCodec or validates it at its schema-defined boundary before transport, with regressions preventing low-level pass-through APIs from reintroducing JSON or unknown codecs.

[rmcdaniel]

Release-critical product-owner correction: this defect violates the active Avro-only SDK contract and must be resolved before this release train is considered qualified. It is P0 for the train even though the implementation is focused. The fix must remain schema-location-aware so customer metadata with codec-like keys is untouched.

[rmcdaniel]

Completed in 0d3abe12f3d3 on main.

Published and verified as 2.0.0-rc.31.

Required public qualification CI (ci.yml) passed in run 31767163161.