Add configurable DLP/PII preflight policy framework for web search

## Summary
Add a foundational DLP/PII preflight policy framework that can inspect user-provided content before SimpleChat sends it to external or component-specific capabilities. The first enforcement target should be web search: when web search is selected by a user or invoked by a workflow, the outgoing search input should be evaluated before it leaves SimpleChat.

## User Value
Admins need a configurable way to reduce the risk of corporate knowledge, private information, or regulated data being sent to external search providers. Users should receive a clear content-safety-style message when a request is blocked, while admins should have enough logging and governance visibility to understand what happened.

## Proposed Behavior
- Provide a reusable policy framework that can be attached to SimpleChat capabilities, starting with web search.
- Evaluate the user's chat prompt or workflow-provided search text before a web search request is sent externally.
- Return a simple enforcement decision from each evaluator: pass, fail, and fail reason.
- When evaluation fails, block the web search call and show a user-facing content safety/governance message.
- Log policy decisions in a way that can align with existing content safety logging and reporting.
- Provide an admin configuration surface, likely connected to governance/admin settings, for enabling policies and assigning them to capabilities.

## Initial Capability Levels
1. Keyword and regex checks
- Admins can define literal keywords, phrases, and regular expressions that should block or flag outgoing web search text.
- The framework should capture a fail reason suitable for user messaging and admin logs.

2. LLM-based analysis
- Admins can select one of their configured GPT deployments/endpoints for policy analysis.
- Admins can write an explicit prompt that instructs the model how to evaluate the outgoing text.
- The model output should be constrained to a pass/fail decision and a fail reason.

3. External analyzer integration
- Add the ability to call an external analyzer service such as Microsoft Presidio Analyzer.
- Admin configuration should support endpoint URL and authentication details.
- Analyzer output should map into the same pass/fail/fail-reason contract used by the framework.

## Acceptance Criteria
- [ ] A reusable policy/evaluator abstraction exists for preflight checks before capability execution.
- [ ] Web search is the first wired capability and cannot send blocked text externally.
- [ ] Keyword and regex policies can be configured and evaluated for web search prompts.
- [ ] The enforcement result supports pass, fail, and fail reason in a consistent contract.
- [ ] Failed checks produce a user-facing content safety/governance message instead of running web search.
- [ ] Policy decisions are logged consistently with, or adjacent to, existing content safety logging/reporting.
- [ ] Admin configuration supports enabling the framework and assigning policies to web search.
- [ ] Design leaves clear extension points for LLM-based analysis and external analyzers such as Presidio.
- [ ] Tests cover allowed search text, keyword block, regex block, failure messaging, and the no-external-call-on-fail path.
- [ ] Documentation explains the initial web search behavior, admin configuration, and future extension model.

## Notes
- Treat this as a foundation for capability-level governance across SimpleChat, not only a web-search-specific filter.
- Explore whether the configuration belongs on the existing governance page, admin settings, or a dedicated policy section.
- This is conceptually similar to custom content safety for outbound capability calls, with a focus on preventing sensitive corporate or private information from leaving the application.
- Future work may expand the same framework to other tools, actions, plugins, and workflow components.