fix(cdk): PDF attachment screening broken — ambient pdf-parse decl describes v1 API against installed v2

Discovered while reviewing PR #673 (#607). **Pre-existing** — landed in #434 (2026-06-30), not introduced by #673.

## Symptom

PDF attachment screening fails for **all** \`application/pdf\` inputs. Every PDF is reported as \`PDF "" could not be processed. It may be corrupt or use unsupported features.\` — indistinguishable from a genuinely corrupt file, so the failure is silent.

## Root cause

\`cdk/src/types/pdf-parse.d.ts\` is an ambient \`declare module 'pdf-parse'\` describing the **v1** API:

\`\`\`ts
function pdfParse(data: Buffer, options?: { max?: number }): Promise;
export = pdfParse;
\`\`\`

The installed package is \`pdf-parse@2.4.5\`, which exports **no callable default**:

\`\`\`
$ node -e "const m=require('pdf-parse'); console.log(Object.keys(m)); console.log(typeof m.default)"
[ 'AbortException', ..., 'PDFParse', 'VerbosityLevel', 'getException' ]
undefined
\`\`\`

The ambient declaration *overrides* v2's real bundled types (\`dist/pdf-parse/cjs/index.d.cts\`), so \`tsc\` stays green against a shape that no longer exists. At runtime, in \`cdk/src/handlers/shared/attachment-screening.ts\`:

- \`pdfParseFn = (mod as any).default ?? mod\` resolves to the module namespace object (no default export).
- Calling \`pdfParseFn(content, { max })\` throws \`TypeError: pdfParseFn is not a function\`, which is caught and rewrapped as the generic "could not be processed" error.

The test at \`cdk/test/handlers/shared/attachment-screening.test.ts:365\` uses a \`{ virtual: true }\` mock supplying \`{ __esModule: true, default: jest.fn() }\` — a v1 shape that no longer exists — so the suite cannot catch this.

## Fix

- Delete \`cdk/src/types/pdf-parse.d.ts\` and let v2's own bundled types apply.
- Rewrite the call site to the v2 API: \`new PDFParse({ data: content }).getText()\` returning a \`TextResult\`.
- Replace the virtual mock with one reflecting the real v2 module (a \`PDFParse\` class with a \`getText()\` method), and add a test that exercises a real (small) PDF end-to-end so a future API mismatch fails loudly.

Commencez par cdk/src/handlers/shared/attachment-screening.ts et cdk/src/types/pdf-parse.d.ts, puis examinez le mock et le test existants dans cdk/test/handlers/shared/attachment-screening.test.ts:365. Vérifiez l’API de pdf-parse@2.4.5 installée, mettez à jour l’appel et le mock pour qu’ils lui correspondent, puis ajoutez un petit test avec un vrai PDF. C’est terminé lorsque les PDF sont examinés avec succès, que les incompatibilités d’API échouent dans les tests et que la déclaration ambient v1 a été supprimée.

Rédigé par le modèle d'indexation à partir du texte de l'issue.