[Test logic] Metadata scenario strips `timestamp` before comparison

[balloob]

Problem

The spec makes metadata.timestamp a required field ("server clock time in microseconds for when this metadata is valid") because the progress-drift formula depends on it.

The conformance harness discards it:

• Client normalizer aiosendspin_client.py:126-148 _normalize_metadata_state has no timestamp key.
• Server expected snapshot aiosendspin_server.py:151-167 _metadata_snapshot has no timestamp key.
• Go server adapters/sendspin-go/server/main.go:648-665 metadataSnapshot has no timestamp key.

A server that omits timestamp or sets it to 0 would pass.

Proposed fix

1. Include timestamp in the client and server metadata summaries.
2. Assert it's present, an integer, > 0, and in microsecond range.
3. Assert server-emitted value equals client-received value exactly.

See sibling issue on the progress-drift formula for the downstream effect of having a trustworthy timestamp.

Spec references

• "Server → Client: server/state metadata object"