feat: cross-stack error taxonomy (#249) + aid_escrow MAX_ALLOWED_TOKENS guard (#233)

[precy41]

Summary

Closes ChainForgee#249 and ChainForgee#233 in one PR.

ChainForgee#249 Cross-stack error taxonomy

• Canonical source of truth: docs/errors.yaml (15 cross-service + 23 Soroban contract identifiers).
• Backend (NestJS): src/common/errors/codes.ts mirrors the YAML; AllExceptionsFilter attaches errorCode on every ErrorResponse. Added optional errorCode field to ApiResponseDto.
• AI service (FastAPI): schemas/error_codes.py mirrors the same enum (extends str for byte-identical JSON). HTTPException / RequestValidationError / AIServiceError / generic handlers and the body-size middleware all emit canonical strings. Fixed latent CODE_BODY_LENGTH_MISMATCH typo in _send_400_mismatch.
• Parity tests: app/backend/src/common/errors/codes.spec.ts and app/ai-service/tests/test_error_codes.py guard both enums against drift.

ChainForgee#233 aid_escrow MAX_ALLOWED_TOKENS guard

• Rust contract: MAX_ALLOWED_TOKENS = 32, new Error::TooManyAllowedTokens = 23 variant, length gate runs before per-token validate_token so a bogus 33rd address can't trigger extra host invocations.
• Backend mapper: soroban-error.mapper.ts covers 19..23 in both the numeric map and the string-fallback map so they never silently fall through to the catch-all 500.
• Tests: Rust covers rejection at length+1, per-token InvalidToken at length=32 (threshold pin), and constant-drift guard MAX_ALLOWED_TOKENS == 32. Backend mapper spec mirrors via numeric and string-fallback tests.

Follow-ups (post-merge)

• Tighten substring matchers in mapContractErrorMessage (includes -> word-boundary anchors).
• Optional dedicated REQUEST_TIMEOUT identifier for HTTP 408 instead of overloading UPSTREAM_TIMEOUT.

Refs ChainForgee#249, ChainForgee#233.