fix(server): propagate --enable-proposed-api in serve-web

[yogeshwaran-c]

What this PR does

Fixes #228781

The code serve-web CLI accepts --enable-proposed-api EXTENSION_ID but never propagated the allowlist to the workbench environment service or the server-side extension scanner. Extensions declaring enabledApiProposals in their package.json therefore had their proposals wiped at runtime by ExtensionsProposedApi and failed to activate with:

Activating extension 'publisher.myextension' failed: Extension 'publisher.myextension' CANNOT use API proposal: chatParticipantPrivate.
Its package.json#enabledApiProposals-property declares:  but NOT chatParticipantPrivate.

Approach

This wires the flag through two paths so it matches desktop behavior:

1. Server scanner (src/vs/server/node/serverServices.ts): extend IProductService.extensionsEnabledWithApiProposalVersion in setupServerServices so the node extension scanner (AbstractExtensionsScannerService) keeps the manifest's enabledApiProposals when --enable-proposed-api is passed.

2. Workbench environment service: add a new enabledExtensionProposedApi field to IWorkbenchConstructionOptions, populate it from the CLI args in webClientServer.ts, and surface it via BrowserWorkbenchEnvironmentService.extensionEnabledProposedApi so the runtime allowlist in ExtensionsProposedApi matches the requested extension IDs.

The existing productConfiguration.extensionsEnabledWithApiProposalVersion propagation in webClientServer.ts is preserved — that field is still needed by the browser-side webExtensionsScannerService for web extensions.

Repro / verification

1. Install an extension declaring "enabledApiProposals": ["chatParticipantPrivate"] in its package.json.
2. code --enable-proposed-api="publisher.myextension" serve-web --port 11001 --without-connection-token --accept-server-license-terms
3. Open the browser URL — extension activates without the proposed-API error.

Without this PR the activation fails as in the issue. With this PR it succeeds, matching the behavior of code desktop with the same flag.

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@bpasero

Matched files:

• src/vs/workbench/browser/web.api.ts
• src/vs/workbench/services/environment/browser/environmentService.ts