fix(agents): preserve protocol precedence when invocations_ws is declared

[Copilot]

Addresses review comment r3601387151 on PR #9009: invocations_ws used an early return in loadServiceProtocol, which silently bypassed the invocations precedence check.

Changes

• nextstep/resolver.go: Add ProtocolInvocationsWS = "invocations_ws" constant
• nextstep/state.go: Replace early return with sawInvocationsWS flag, mirroring the existing sawInvocations pattern — enforces responses > invocations > invocations_ws priority regardless of declaration order
• nextstep/state_test.go: Add test cases for invocations_ws alone and for precedence over invocations_ws when combined with responses or invocations

// Before — early return skips invocations check
case ProtocolInvocationsWS:
    return ProtocolInvocationsWS

// After — flag approach preserves precedence
sawInvocationsWS := false
...
case ProtocolInvocationsWS:
    sawInvocationsWS = true
...
if sawInvocations {
    return ProtocolInvocations
}
if sawInvocationsWS {
    return ProtocolInvocationsWS
}

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