feat(ai-provider): route provider HTTP through proxy-aware net.fetch

[DeJeune]

What this PR does

Before this PR:

• AI provider HTTP requests ran on Node's globalThis.fetch (undici). The app proxy reached them only through the undici global-dispatcher patch in NodeProxyController, and provider-specific fetch wrappers (e.g. CherryAI request signing) called the global fetch directly.

After this PR:

• A composable customFetch built on Electron net.fetch is added and installed as the default base fetch for every provider config in providerToAiSdkConfig (settings.fetch ??= customFetch). net.fetch runs on Chromium's network stack, so requests honour the app proxy configured by ProxyManager via session.setProxy() / app.setProxy().
• Provider-specific wrappers compose on top of customFetch instead of being replaced: the CherryAI signing wrapper now calls customFetch, and the dev-mode HTTP-trace wrapper layers above it — the proxy-aware net.fetch stays the innermost network call in every case.

N/A

Why we need it and why it was done in this way

Routing AI provider traffic through net.fetch makes the configured app proxy apply on Chromium's network stack (PAC / SOCKS / auth handled natively, consistent with the rest of the app's net.fetch usage), rather than depending solely on the symbol-based undici dispatcher patch.

The following tradeoffs were made:

• customFetch is the innermost base; higher layers (provider signing, HTTP trace) wrap it. Installed via ??= so a provider that ships its own fetch wrapper is preserved, not clobbered.

The following alternatives were considered:

• Relying only on the undici global-dispatcher patch — fragile (depends on internal undici symbols) and not consistent with the app's session proxy.
• Unconditionally setting providerSettings.fetch = customFetch — would clobber provider wrappers such as CherryAI request signing.

Links to places where the discussion took place: N/A

Breaking changes

None.

Special notes for your reviewer

The custom image transports (dmxapi / ppio / ovms) that call bare fetch(...) directly are intentionally out of scope — they still proxy via the undici dispatcher patch. This PR unifies the provider-config fetch path only.

Checklist

This checklist is not enforcing, but it's a reminder of items that could be relevant to every PR.
Approvers are expected to review this list.

• Branch: This PR targets the correct branch — main for active development, v1 for v1 maintenance fixes
• PR: The PR description is expressive enough and will help future contributors
• Code: Write code that humans can understand and Keep it simple
• Refactor: You have left the code cleaner than you found it (Boy Scout Rule)
• Upgrade: Impact of this change on upgrade flows was considered and addressed if required
• Documentation: A user-guide update was considered and is present (link) or not required. Check this only when the PR introduces or changes a user-facing feature or behavior.
• Self-review: I have reviewed my own code (e.g., via /gh-pr-review, gh pr diff, or GitHub UI) before requesting review from others

Release note

AI provider requests now route through Electron net.fetch so the configured app proxy (system/custom, including SOCKS) reliably applies to model traffic.