Add error path and edge case tests for envelope package

[lanycrost]

Problem statement

The envelope package has 78.8% statement coverage with 4 tests, but all error paths and several important edge cases are untested.

Uncovered error paths (0 hits in coverage)

• Marshal(nil) — line 77-79
• Unmarshal("") (empty payload) — line 92-94
• Unmarshal(invalidJSON) — line 96-98
• FromBinaryFrame(nil) — line 124-126
• FromBinaryFrame with invalid JSON payload — line 131-133
• ToBinaryFrame when Marshal fails — line 108-110

Untested edge cases

• Empty envelope (&Envelope{}) with no fields set
• Nil vs empty Payload (nil vs json.RawMessage(nil) vs json.RawMessage("null"))
• Kind defaulting: no default when no payload/inputs/transports
• Kind defaulting: triggered by Inputs alone, or Transports alone
• Explicit Version is preserved, not overwritten
• Timestamp conversion: int64 → uint64 with negative values in ToBinaryFrame
• Timestamp conversion: uint64 → int64 overflow in FromBinaryFrame

Untested types

• HookPayload — zero test coverage
• TransportDescriptor — only tested as part of round-trip, no isolated tests

Proposed change

Add the following test functions to envelope_test.go:

// Error paths
TestMarshal_ReturnsError_WhenEnvelopeIsNil
TestUnmarshal_ReturnsError_WhenPayloadIsEmpty
TestUnmarshal_ReturnsError_WhenPayloadIsInvalidJSON
TestFromBinaryFrame_ReturnsError_WhenFrameIsNil
TestFromBinaryFrame_ReturnsError_WhenPayloadIsInvalidJSON

// Edge cases
TestMarshal_DoesNotDefaultKind_WhenNoPayloadInputsOrTransports
TestMarshal_DefaultsKind_WhenOnlyInputsPresent
TestMarshal_PreservesExplicitVersion
TestToBinaryFrame_WithNegativeTimestamp
TestFromBinaryFrame_WithMaxUint64Timestamp

// HookPayload
TestHookPayload_MarshalUnmarshal_RoundTrip
TestHookPayload_WithNilData

Affected area

• Proto definitions
• Envelope helpers

Compatibility / migration

Test-only changes. No behavior changes.

Additional context

Current coverage: 78.8%. These tests would bring it above 95% and cover all error return paths. Identified during QA review.