Add event-level redact_event hook, protect server/client identity fields

[naji247]

Summary

• Adds an event-level redact_event hook (AgentCatOptions.redact_event), matching the redactEvent/RedactEvent hooks already available in the TypeScript and Go SDKs. It runs before redact_sensitive_information, receives the full event, and may return a modified event or None to drop it entirely. id, session_id, project_id, event_type, and timestamp are restored from the original event afterward regardless of what the hook returns.
• Adds server_name, server_version, client_name, and client_version to PROTECTED_FIELDS: these are system-reported MCP metadata, not user-supplied content, and should always reach the dashboard intact rather than passing through redact_sensitive_information.

Example

from agentcat import AgentCatOptions

def redact_event(event):
    if event.resource_name == "get_credentials":
        return None
    if event.resource_name == "export_report":
        event.response = None
    return event

agentcat.track(server, "proj_0000000", AgentCatOptions(redact_event=redact_event))

Test plan

• Full test suite passes
• New tests covering the event-level hook: raw-value visibility, mutation, drop, restored fields, hook failures, sync/async, and pipeline ordering relative to redact_sensitive_information
• README updated with usage docs