[MetaScheduler] Improve print info about builder/runner state

[cbalint13]

Adds on-screen info about samples failed to build, that may happen in early stages.

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Before:

2026-06-14 18:36:30 [INFO] [task_scheduler.cc:196] TaskScheduler picks Task #0: "dense"
2026-06-14 18:36:32 [INFO] [task_scheduler.cc:209] Sending 64 sample(s) to builder
2026-06-14 18:38:47 [INFO] [task_scheduler.cc:211] Sending 64 sample(s) to runner

After:

2026-06-14 19:53:23 [INFO] [task_scheduler.cc:193] TaskScheduler picks Task #0: "dense"
2026-06-14 19:53:25 [INFO] [task_scheduler.cc:206] Sending 64 sample(s) to builder
2026-06-14 19:55:39 [INFO] [task_scheduler.cc:215] Build errors: 64 sample(s)
2026-06-14 19:55:39 [INFO] [task_scheduler.cc:217] Sending 0 valid sample(s) to runner

[gemini-code-assist]

Code Review

This pull request modifies the task scheduler to count and log build errors before sending samples to the runner. The review feedback points out an efficiency issue where task->builder_results.value() is repeatedly called inside a loop, and suggests extracting it to a local reference variable before the loop to avoid redundant unwrapping and reference counting.

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