Stop grpc streams when thresholds are met

What does this MR do and why?

This MR fixes gRPC message size errors by enforcing result limits at the stream level, preventing individual Zoekt nodes from sending responses that exceed gRPC message size limits.

Problem

Search requests were failing with too large messages, ie: grpc: received message larger than max (26588986 vs. 16777216).

The issue occurred because limits (max_line_match_results, max_line_match_results_per_file) were only enforced in multiNodeSearch() after receiving and aggregating results from all streams. A single Zoekt node could accumulate 25,000+ line matches and send a 26MB+ gRPC message to the webserver before any limits were applied, exceeding the default 16MB gRPC limit.

Solution

Enforce max_line_match_window and max_line_match_results_per_file while receiving the gRPC stream in handleGrpcSearchStream():

• Truncate each file to max_line_match_results_per_file matches as files are processed
• Stop reading the stream immediately when max_line_match_window line matches are collected
• Uses proto.Clone() to safely truncate protobuf messages

This prevents any single stream from exceeding gRPC message limits, while maintaining correct behavior: final sorting by relevance score still occurs after combining results from all nodes.

Example: With max_line_match_window: 5000, each stream now stops at 5000 line matches instead of potentially sending 25,000+, keeping messages well under the gRPC limit.

How to set up and validate locally

Turn off gdk webservers and turn on webservers with these changes in different terminal panes.

In gdk directory:

gdk stop gitlab-zoekt-webserver-development-1 gitlab-zoekt-webserver-development-2

In this repo's directory:

make gdk-webserver-1

make gdk-webserver-2