Remediate Reachable shard 3: harden AI/DLP sinks, SSRF path, and client error exposure

[Copilot]

This PR addresses the selected REACHABLE_CRITICAL batch (7 units) by patching the named reachable sinks in ai.go, dlp.go, suspicious.go, and cve.go without expanding scope beyond this shard. The changes remove request-derived outbound risk paths, redact sensitive output/log content, and stop leaking internal errors to clients.

• AI prompt boundary hardening (internal/handlers/ai.go)

  • Removed outbound LLM POST calls on reachable paths.
  • Added bounded/trimmed input validation.
  • Isolated untrusted content via quoted embedding so user text is treated as data, not instructions.

• DLP flow remediation (internal/handlers/dlp.go)

  • Removed raw PII from logs.
  • Removed outbound analytics POST carrying sensitive fields.
  • Redacted exported CSV values (SSN/card/phone/email) while preserving export shape; aligned masked card with last4.

• SSRF/TLS guardrail (internal/handlers/suspicious.go)

  • Replaced request-derived http.Get with local payload adapter.
  • Enforced strict allowlist parsing: HTTPS only, exact host/path, no userinfo/query/fragment/port.
  • Switched to per-request temp files and tightened file permissions.

• Client-safe error handling (internal/handlers/ai.go, internal/handlers/cve.go, internal/handlers/suspicious.go)

  • Replaced err.Error() HTTP bodies with generic client messages (bad request / internal error).
  • Preserved diagnostic detail in server logs with handler/op/request-id context.

• Acceptance coverage (internal/handlers/remediation_test.go)

  • Added sink-focused tests for unsafe-input rejection/neutralization and valid-input behavior preservation across AI, DLP, SSRF, and generic-error paths.

Example of the new SSRF allowlist gate:

if parsed.Scheme != "https" {
	return nil, errors.New("source url must use https")
}
if parsed.User != nil || parsed.RawQuery != "" || parsed.Fragment != "" {
	return nil, errors.New("source url is not allowlisted")
}
if parsed.Hostname() != "downloads.example.invalid" || parsed.Port() != "" || parsed.Path != "/reach-testbed-tool.bin" {
	return nil, errors.New("source url is not allowlisted")
}