[CF-DATAPLANE][CPEX] Support plugin-directed tool execution actions

[lucarlig]

Parent epic: #4525

Gap

Rust CPEX tool hooks can currently deny or transform pre-call arguments and post-call results. Full tool handling also needs bounded execution controls emitted by plugins.

Scope

Support validated CPEX actions for:

• deny/allow and argument/result transformation or redaction;
• allowlisted upstream header injection/removal;
• timeout overrides within operator-defined limits;
• explicit retry directives constrained by the tool-call retry policy;
• failure directives consistent with each binding's on_error policy.

Apply actions in a deterministic order and reject contradictory or security-sensitive actions.

Acceptance criteria

• Actions compose predictably across multiple ordered bindings.
• Plugins cannot alter authentication, hop-by-hop, host/routing, or other protected headers.
• Timeout and retry actions cannot exceed operator limits or make non-idempotent retries implicit.
• Denied calls never reach the backend.
• Post-hook redaction occurs before the result is returned or recorded.
• Tests cover action ordering, conflicts, protected headers, deny, transform, retry, timeout, and failure handling.