fix: unpaired-surrogate attachment names no longer break Gmail sync

[charan-rathore]

What changed

Two places in packages/integrations/src/google.ts run encodeURIComponent on attachment names. A name with an unpaired surrogate makes it throw "URI malformed":

1. Incoming (mapMessage): one sender's bad attachment filename breaks the whole inbox listing on every refresh.
2. Outgoing (the GoogleClient MIME builder): a bad attachment name makes the send fail.

Both now replace unpaired surrogates with U+FFFD before encoding. I used a small local wellFormed helper rather than String.prototype.toWellFormed(), because the repo targets ES2023 where toWellFormed does not type-check (TS2550). It is the Gmail-path counterpart of #117, which fixes the truncation side in apps/server/src/files.ts; the two do not overlap. Open #97 also edits google.ts and tests/google.test.ts for different parsing issues, so a rebase may be needed if it lands first. No issue was filed.

Verification

• tests/google.test.ts: the two new tests fail on unchanged main with "URI malformed" (25 pass, 2 fail); patched 27/27.
• npx tsc --noEmit and npx biome check on both changed files are clean.
• Full tests/*.test.ts: 222 pass, 4 fail. All 4 are in tests/browser.test.ts, which fails the same way on an unpatched base.

Integration limits

No live Gmail account was used; the tests exercise the mapping and MIME builder directly.