FIX: Bound backend request sizes and field lengths

[varunj-msft]

Description

The backend accepted request bodies, strings, lists, and maps of any size, silently dropped label filters without a key:value separator, and returned 500 for turn filters too large for the database.

• middleware/request_size.py: the backend reads at most 100 MiB of a request body (413, from Content-Length or while streaming) and rejects URLs over 8 KiB (414). Both 413 paths return the same RFC 7807 problem response (RequestTooLargeError plus a handler in register_error_handlers). The middleware is registered innermost so the streamed 413 reaches the route as an HTTPException; CORS stays outside it so browsers can read 413/414.
• models/common.py: limits applied to request models and route path/query parameters (identifiers 256, cursors 1024, free text such as objectives 100,000, labels up to 100 entries with keys 128 and values 1024, lists 100, configuration and initializer file contents 1,048,576 characters). Prompt content (message pieces, system prompts, converter preview input) and free-form values (metadata and parameter values) are limited only by the body size so long prompts and base64 media keep working.
• The limits shared with scenario requests (list items, identifiers, label keys and values) are defined once in pyrit/models/request_limits.py (per review). pyrit/models/catalog/scenario.py uses them for run and estimate requests, and backend/models/common.py imports them; body, URL, cursor, and text limits stay in the backend. Technique tokens allow converter modifiers (technique:converter.<name>:...) up to 4,096 characters.
• Label filters are validated by validate_label_filter: a : separator is required and the stripped key and value must fit the label limits (422 with a readable message); parse_label_query_params reuses it instead of dropping malformed filters.
• min_turns / max_turns get an upper bound of 10,000 and cutoff_index must be non-negative.
• README "Request Limits" subsection.

Coordination:

• FEAT: Async manual sends #2861 (main.py, README) and [BREAKING] MAINT: Make backend scenario reads async-only #2953 (scenario route bodies) currently merge cleanly with this PR.
• [BREAKING] FEAT: Make scenario dataset sources and limits explicit #2956 and FEAT: Add ScenarioPreset #2931 also edit pyrit/models/catalog/scenario.py: [BREAKING] FEAT: Make scenario dataset sources and limits explicit #2956 changes max_dataset_size next to fields bounded here and adds tests to tests/unit/models/test_scenario_request.py, and FEAT: Add ScenarioPreset #2931 changes the adjacent max_dataset_size description. Whichever lands second resolves a few field-line conflicts (keep [BREAKING] FEAT: Make scenario dataset sources and limits explicit #2956's dataset-limit type next to this PR's bounded types).
• FEAT: Add conversation editing and shared save workflows #2853 conflicts with this PR in pyrit/backend/models/attacks.py.
• The other PRs from this story (FIX: Keep backend media and upload inputs inside managed storage #2958, FIX: Accept only supported external inputs when building components #2959, FIX: Register created converters and targets only after the response is built #2964) merge cleanly with this one.

Tests and Documentation

• tests/unit/backend/test_request_size.py (declared and streamed 413 through the real app with identical bodies), test_common_models.py, test_api_routes.py, tests/unit/models/test_scenario_request.py.
• python -m pytest tests/unit/backend tests/unit/models -n 4 — 3562 passed, 4 skipped.
• ruff format / ruff check — passed; ty check pyrit/ — no new diagnostics; pre-commit hooks pass.
• build_and_test workflow run on the fork at 7c7afc8: pre-commit on ubuntu/windows/macOS and make unit-test-junit on ubuntu/windows/macOS x Python 3.11-3.14 x dev/dev_all — all 35 jobs passed.
• Live backend at 92fe5c7 (the follow-up commit only moves the shared limits; the values are unchanged): 25/25 checks (413 declared and streamed over a real socket with the same problem body, 414, 422 limits including label key/value limits, a 54k-character objective, a ~7 MB base64 image and a 200k-character text piece accepted, CORS preflight, request-id and security headers on 413).
• Docs: README "Request Limits". No notebooks changed.