What this PR does

Before this PR:
WebSearchSource was defined as a TypeScript enum.

After this PR:
WebSearchSource is converted to an as const object (WEB_SEARCH_SOURCE) with a Zod schema (WebSearchSourceSchema) and a derived type (WebSearchSource).

Part of #12846 — split out as a separate PR to reduce the number of files to review in the final refactor PR.

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

This aligns WebSearchSource with the project's existing convention of using as const objects instead of enums (e.g., EFFORT_RATIO, FILE_TYPE). The Zod schema integration also enables runtime validation.

The following tradeoffs were made:

• The value object is renamed to WEB_SEARCH_SOURCE (UPPER_SNAKE_CASE) to follow the const object convention, while the type name WebSearchSource is preserved for backward compatibility.

The following alternatives were considered:

• Keeping the enum — rejected because it diverges from the project's established pattern.

Breaking changes

None. All runtime string values remain identical. This is a purely internal refactor.

Special notes for your reviewer

• This is a mechanical rename across 16 files with no behavioral changes.
• pnpm build:check passes (lint + 3026 tests + typecheck).
• One pre-existing commented-out line in AiSdkToChunkAdapter.ts:35 still references the old WebSearchSource name — can be cleaned up separately.

Checklist

• 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: Not required (internal refactor only)

Release note

NONE