Fix: validate $filter before streaming response (return 400 not malformed 200)

[davetha]

Summary

When $filter references an unknown property (e.g. $filter=BadField eq 'value'), Lodata starts streaming the HTTP 200 response before evaluating the filter. The filter parse error then occurs mid-stream, injecting an error JSON fragment into the already-started response body. The client receives malformed output that is neither a valid OData error response (HTTP 400) nor valid JSON.

Per the OData specification, an invalid $filter should return HTTP 400 Bad Request before any response body is sent.

Changes

Added assertValidFilter() in EntitySet::get(), called immediately after the existing assertValidOrderBy() and before the StreamedResponse callback is registered. This eagerly parses the filter expression using the entity set's own filter parser, causing any ParserException (which extends BadRequestException) to be thrown before streaming begins.

How it works

The call chain for a GET request is:

1. get() — validates orderby, now validates filter, then registers the streaming callback
2. Streaming callback → emitJson() → query() → generateWhere() → generateTree()

With this fix, an invalid $filter throws at step 1, before StreamedResponse::sendContent() begins writing. The exception propagates through Laravel's normal error handling and produces a clean HTTP 400.

Impact

• Invalid $filter now returns proper HTTP 400 instead of malformed HTTP 200
• Mirrors the existing assertValidOrderBy() pattern exactly
• Valid requests parse the filter tree twice (once for validation, once in generateWhere()) — negligible cost for correctness
• No new exception types; ParserException already maps to HTTP 400

Testing

• PHP syntax check passes
• Pattern follows existing assertValidOrderBy() precedent