Validate mitmweb flow filters on the backend - #8331

Description

This PR moves mitmweb flow-filter validation (search/highlight/intercept) from the frontend to the backend, making mitmproxy.flowfilter the single source of truth for filter parsing.

This adds a GET /filter/validate?expression=... endpoint that returns either the parsed filter description or a validation error.

The filter input now:

• validates filters through the backend;
• waits 300ms after typing before sending a request;
• cancels obsolete requests and ignores stale responses;
• only propagates filters after successful validation;
• displays backend validation and request errors.

The obsolete frontend flow-filter parser (filt.js and filt.peg) has been removed.

Note

For a valid expression, flowfilter.parse() is called twice: once by the validation endpoint and again when the filter is applied by Search, Highlight, or Intercept.

This is intentional. The validation request and the operation applying the filter are independent, and the second parse keeps the existing backend paths authoritative without sharing parser objects or introducing a cache. The additional cost should be negligible.

It closes also: #8239

Checklist

• I have updated tests where applicable.
• I have added an entry to the CHANGELOG.