ci: increase llmobs parallelism to 5 (#15317)

## Description

We are seeing some timeouts in CI, the timeouts don't seem related to
number of venvs run per-job, but there are exactly 5 venvs so might as
well do 1 per-job anyways.

## Testing

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## Risks

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-->

## Additional Notes

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Author: brettlangdon

tests/llmobs/suitespec.yml

@@ -114,6 +114,7 @@ suites:
       - tests/llmobs/*
     runner: riot
     snapshot: true
+    parallelism: 5
   mcp:
     parallelism: 1
     paths: