[bug] it is not possible to use context:0 ... N

[mpet]

Describe the bug

We only wanted to see a summary for matching lines.

When using context:0 ... N in the search field
we get a crash in webserver.

To reproduce

I tried:

Helpers context:0

or

Helpers context:1

Both failed.

Sourcebot deployment information

Sourcebot version:

Current version: v5.0.3(496b09a)

Additional information

No response

[rachit367]

Picking this up. Root cause: context:<value> parses as a named search-context lookup (ContextExpr), and when the context doesn't exist, onExpandSearchContext in features/search/parser.ts throws a plain Error instead of a ServiceErrorException. Only SyntaxError is caught and mapped to a clean response, so the plain error propagates and crashes the request. Fixing it to throw a ServiceErrorException with the existing SEARCH_CONTEXT_NOT_FOUND code so it returns a graceful 404.