fix(schema): coerce string protocolVersion in InitializeRequest

[WeeeeeKdY]

Problem

Some ACP clients send a date string like "2024-11-05" as protocolVersion instead of an integer. Zed is a known example — its agent_servers implementation can emit the MCP-style date format during the ACP handshake.

The current Python SDK rejects this outright with a pydantic validation error, causing the agent process to crash immediately after the client connects:

acp.exceptions.RequestError: Invalid params
  protocol_version: Input should be a valid integer, unable to parse string as an integer

Root cause

The Rust SDK (agent-client-protocol-schema) already handles this gracefully with an explicit comment in the source:

fn visit_str<E>(self, _value: &str) -> Result<Self::Value, E> {
    // Old versions used strings, we consider all of those version 0
    Ok(ProtocolVersion::V0)
}

The Python SDK's generated schema.py uses a strict pydantic int type with no equivalent fallback.

Fix

• Add a field_validator("protocol_version", mode="before") to InitializeRequest in src/acp/schema.py that coerces non-integer values to 1 (current stable version), mirroring the Rust SDK's lenient behaviour. Strings that look like integers (e.g. "1") are parsed normally; arbitrary strings (e.g. "2024-11-05") fall back to 1.
• Add a CLASS_VALIDATOR_INJECTIONS table and _inject_field_validators post-processing step to scripts/gen_schema.py so the validator is automatically re-injected on future schema regenerations.
• Add _ensure_pydantic_import helper to add field_validator to the generated pydantic import line when needed.

References

• Rust SDK lenient deserializer: https://github.com/agentclientprotocol/rust-sdk/blob/main/crates/agent-client-protocol-schema/src/version.rs