fix(core): honor config requestOptions in ContinueServerClient

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Description

Config-sync and index-cache requests in ContinueServerClient used the global fetch(), so config.yaml requestOptions (proxy, verifySsl, caBundlePath) were ignored on those paths. Behind an HTTPS-intercepting corporate proxy with a custom CA, this surfaces as a startup "Connection error" in core.log even though LLM requests work fine (they already go through fetchwithRequestOptions).

This routes the client's requests through fetchwithRequestOptions and passes config.requestOptions in from CodebaseIndexer, so the same proxy/TLS settings apply to config sync and index caching.

Fixes #12956

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

N/A — network-path change with no UI. Any request made by ContinueServerClient previously bypassed config.yaml requestOptions; after this change it uses them.

Tests

Added core/continueServer/stubs/client.vitest.ts — verifies getConfig and getFromIndexCache forward the constructor's requestOptions to fetchwithRequestOptions.

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