fix(static-config): extract mergeConfig fields

[leno23]

Summary

• Teach the static Vite config extractor to understand mergeConfig(...) calls.
• Allow defineConfig(mergeConfig(...)) and mergeConfig({}, defineConfig({...})) to stay statically analyzable when their merged inputs are static enough.
• Preserve conservative behavior for spreads, computed keys, and non-static fields so dynamic config still falls back when needed.

Closes #1709

Test Plan

• git diff --check
• RUSTUP_TOOLCHAIN=stable rustfmt --edition 2024 --check crates/vite_static_config/src/lib.rs

Attempted but blocked locally:

• RUSTUP_TOOLCHAIN=stable cargo test -p vite-static-config merge_config -- --nocapture failed during workspace dependency resolution because this workspace needs nightly -Z bindeps for fspy from vite-task.
• Repo nightly toolchain (nightly-2026-06-10) is partially installed locally and rustup stalled while downloading the required components.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve nested fields when merging configs

Here the static merge treats a later top-level field as a full replacement, but Vite's mergeConfig recursively merges object fields (the existing packages/cli/src/__tests__/define-config-test-field.spec.ts test expects both nested test keys to survive). For mergeConfig({ run: { tasks: { build: { command: '...' } } } }, { run: { cacheScripts: false } }), this returns run with only cacheScripts; packages/cli/binding/src/cli/handler.rs then accepts that Json value without NAPI fallback, so configured tasks from the base config disappear at runtime.

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[fengmk2]

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