Fix ProxyServer default to allow browser context proxy inheritance

[Copilot]

Browser contexts created with default options were explicitly setting ProxyServer to empty string, overriding the browser's proxy configuration (including OS proxy settings). Pages created directly via IBrowser.NewPageAsync would respect proxy settings, but pages in default browser contexts would not.

Changes

lib/PuppeteerSharp/Cdp/CdpBrowser.cs

• Removed ?? string.Empty default for ProxyServer parameter in CreateBrowserContextAsync
• When null, the property is now omitted from CDP message (via JsonIgnoreCondition.WhenWritingNull), allowing proxy inheritance

// Before: Explicitly overrides proxy to empty
ProxyServer = options?.ProxyServer ?? string.Empty,

// After: Allows browser proxy inheritance when unspecified
ProxyServer = options?.ProxyServer,

.gitignore

• Added testKey.pem to excluded test certificate files

Behavior

• Default context: CreateBrowserContextAsync() → inherits browser/OS proxy
• Explicit proxy: CreateBrowserContextAsync(new() { ProxyServer = "..." }) → uses specified proxy (unchanged)
• Explicit null: CreateBrowserContextAsync(new() { ProxyServer = null }) → inherits browser/OS proxy

Aligns with upstream Puppeteer TypeScript implementation which passes proxyServer without defaulting.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Default options of BrowserContextOptions doesn't inherit the browser's ProxyServer</issue_title>
<issue_description>Pages created from IBrowser.NewPageAsync will pick up the OS' proxy server.

Using a browser context created with default options, pages created with IBrowser.NewPageAsync won't pick up the proxy server from the OS.

This was tested with Chrome on Windows.

I suspect that defaulting the BrowserContextOptions.ProxyServer to string.Empty would cause the proxy value to be overridden.</issue_description>

<agent_instructions>CreateBrowserContextAsync should default to null not to string empty</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Custom agent used: Bidi migration
Implement bidi code

• Fixes Default options of BrowserContextOptions doesn't inherit the browser's ProxyServer #2916

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