perf(pdf-server): share cache across server instances and dedupe form parsing

[ochafik]

Summary

Three perf fixes for the pdf-server when run behind a stateless HTTP transport (where createServer() is invoked on every request):

• Module-level sharedPdfCache — createPdfCache() is now called once at module load and reused by every server instance, instead of being created (and immediately discarded) per request. This lets read_pdf_bytes chunk reads hit the cache across requests, avoiding repeated full-body downloads when the upstream origin doesn't honor Range.
• Memoized mcp-app.html — the ~4 MB viewer HTML is read from disk once (cachedAppHtml ??= ...) instead of on every resource read.
• Single download + parse in display_pdf — extractFormSchema and extractFormFieldInfo now accept an already-parsed PDFDocumentProxy. display_pdf fetches the bytes once, calls pdfjs.getDocument() once, passes the doc to both extractors, and destroy()s it in a finally. Previously each extractor independently downloaded the full PDF and parsed it.

Behavior note

Both extractors now run inside a single try block, so if schema extraction throws, field-info extraction is also skipped (previously each had its own try/catch). Risk is low: the only throw point inside extractFormSchema (getFieldObjects) is already caught internally and returns an empty schema rather than propagating.

Tests

bun test examples/pdf-server — 177 pass, 0 fail, 1 pre-existing skip. Build (tsc + vite + bun bundle) and prettier check both clean.

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commit: 4702eb6