fix(acp): allow v2 initialize with future params

[benbrandt]

Summary

• treat v2-to-v1 initialize projection as an optional connection-reuse probe, so valid v2 traffic is not rejected when projection is unavailable
• reuse a fallback connection only when the projected v2 parameters exactly match the v1 implementation; otherwise reconnect and initialize cleanly
• preserve raw extension fields for exact-version requests while canonicalizing future protocol versions through the selected schema
• remove SDK tests and migration guidance that depend on the experimental schema-wide v2::conversion helpers

Design

ACP traffic remains version-specific after initialization. The private adapter covers only the initialize negotiation boundary, where the router has enough context to define the mapping. This prepares the SDK for the upstream removal of broad v1/v2 conversion helpers without making conversion failure a protocol failure.

Verification

• just test
• cargo clippy -p agent-client-protocol --all-targets --all-features -- -D warnings
• cargo check -p agent-client-protocol --no-default-features --features unstable_protocol_v2
• focused initialize-normalization and protocol-v2 tests