fix: handle FDv2 intentCode none as listening, reject unknown codes

[beekld]

Bring the FDv2 protocol handler's server-intent dispatch in line with the spec and the Java implementation.

• intentCode: none keeps the handler in listening state so a subsequent partial cycle works.
• Unrecognised intent codes return a protocol error instead of being collapsed with none.

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Note

Medium Risk
Changes streaming protocol state machine behavior for server-intent, which can affect how clients process FDv2 payloads; scope is limited to intent dispatch with new tests.

Overview
Aligns server-intent handling in FDv2ProtocolHandler with the FDv2 spec and Java client.

For intentCode: none, the handler still emits an immediate empty kNone changeset, but it now leaves internal state in partial/listening mode instead of inactive, so a later put-object / payload-transferred cycle can produce a partial changeset.

Unrecognized intent codes (deserialized as kUnknown) are no longer treated like none: the handler resets and returns a ProtocolError with "server-intent had an unrecognized intent code".

Tests cover a partial cycle after none and protocol errors for unknown codes (e.g. future-code).

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