fix(google): guard null parts in planGoogleHistory user-turn scan

[dex0shubham]

Problem

planGoogleHistory (src/core/google.ts) scans contents for user turns:

c.parts.some((p) => typeof p.text === 'string' && p.text.trim())

p.text is dereferenced without guarding p. A null element inside a contents turn's parts throws TypeError: Cannot read properties of null (reading 'text'). The proxy catches it and returns a 502 "pxpipe transform failed", so the whole Gemini request fails instead of being forwarded.

This violates the module's own contract: every other part-iterating function guards null (googleHistoryUnit: if (!part) { … continue }), and malformed request shapes are supposed to pass through unchanged. The existing test already pins systemInstruction: { parts: [null] } as pass-through, but the contents path — the one that reaches this scan — was untested.

Fix

Add the missing p && guard, matching the module's existing null-tolerance:

c.parts.some((p) => p && typeof p.text === 'string' && p.text.trim())

Extended the existing malformed-passthrough test with { contents: [{ role: 'user', parts: [null] }] }. Verified it fails on the old code (TypeError) and passes with the fix.

Full suite green (1204/1204); typecheck clean.