feat(filter): configure and validate outbound subrequest filter chains

[leseb]

Parent: #1072

Depends on #1073.

Summary

Callers of the filtered-subrequest executor need a safe way to select and build outbound filter chains through the same registry and lifecycle as their containing pipeline.

Add registry-aware outbound-chain configuration and validation without turning SubRequestClient into a filter orchestrator.

Scope

• Allow a callout-capable filter to bind a configured outbound filter chain during pipeline construction.
• Resolve the chain through the active FilterRegistry, including application-registered filters.
• Rebuild outbound chains as part of normal pipeline reload and publish the new configuration atomically.
• Define explicit projection of approved parent request data into an isolated child context.
• Validate missing filters, filter ordering, recursive chain references, and maximum nesting depth before activation.
• Preserve existing destination authority, DNS/SSRF, TLS/SNI, Host, deadline, and transport-limit enforcement.
• Document the configuration and lifecycle contract for downstream callers.

Acceptance Criteria

• Callers receive a prebuilt outbound pipeline rather than parsing or resolving filter configuration per request.
• Application-owned filters can be resolved from the active registry.
• Invalid references, cycles, excessive nesting, and invalid ordering fail configuration validation.
• Failed reload validation does not publish a partial mix of parent and outbound pipelines.
• Child contexts inherit no headers or extensions by default.
• Only explicitly approved request-scoped values can be projected into a child context.
• Credentials cannot be disclosed before the outbound destination has been resolved and validated.
• Configuration and lifecycle behavior have focused unit and reload tests.

Out of Scope

• Implementing application-specific credential policy.
• Migrating Praxis AI callout clients.
• Automatically forwarding incoming headers or all request extensions.