Fix proxy headers leak errconnect on strict proxies

[joelwizard]

This pull request adds a comprehensive test suite for the proxyPolicy to ensure it does not leak application-level headers into proxy connections and correctly handles proxy and no-proxy scenarios. The tests cover both HTTP and HTTPS requests, custom agents, and proxy bypass logic.

Tests for header leakage and proxy behavior:

• Adds tests to verify that proxyPolicy does not leak application headers (such as Content-Type, x-ms-version, etc.) into the CONNECT request when using HttpsProxyAgent, addressing issues with strict corporate proxies.
• Verifies that both HttpsProxyAgent and HttpProxyAgent are used appropriately for HTTPS and HTTP requests, respectively, and that the correct proxy URL is set.
• Ensures that hosts specified in the no_proxy list are correctly bypassed and do not use a proxy agent.
• Confirms that a custom agent already set on the request is not overwritten by the policy.

Fixes #747

[Copilot]

Pull request overview

This PR addresses failures with strict corporate proxies by ensuring application-level request headers are not forwarded into proxy agent CONNECT headers, and adds a Jest test suite validating proxy behavior and no-proxy/custom-agent scenarios (Fixes #747).

Changes:

• Stop passing request.headers into HttpProxyAgent/HttpsProxyAgent constructor options in the bundled proxyPolicy implementation to prevent header leakage into CONNECT.
• Add __tests__/proxy-policy.test.ts covering header leakage prevention, proxy routing, no-proxy bypass, and preserving pre-set custom agents.

Reviewed changes

Copilot reviewed 1 out of 3 changed files in this pull request and generated 3 comments.

File	Description
dist/upload/index.js	Removes forwarding of application headers into proxy agent options within the bundled proxy policy.
dist/merge/index.js	Same proxy header leakage fix applied to the merge bundle.
tests/proxy-policy.test.ts	Adds regression tests for proxy policy behavior (header leakage, proxy/no-proxy, agent selection, custom agent preservation).

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