fix: stop rejecting valid NL-search SQL that contains the word error

[pujitha24]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Natural language search returns a false "query is invalid" error (rendered as an HTTP 500) for queries like "Which pods are in error state", while other queries succeed.

The root cause is in ConvertTextToSQL (pkg/core/manager/ai/search.go): it validated the LLM's raw response with IsInvalidQuery, which flagged the response as invalid whenever it contained the case-insensitive substring "error" anywhere in the text. A query about pod error state legitimately produces a generated SQL statement whose WHERE clause itself contains the word "error" (matching a Kubernetes pod status value), so this substring check rejected perfectly valid SQL as invalid.

ConvertTextToSQL now uses "was a SELECT statement extracted from the response" as the sole validity signal, since the prompt (prompt.go, Text2sqlType) already instructs the model to always emit SQL starting with select * from and ending with ;, or the literal word "Error" when it cannot produce a query. The now-unused IsInvalidQuery helper and its test were removed since nothing else in the repo referenced it.

Validation:

• go build ./pkg/core/manager/ai/... passes.
• go test ./pkg/core/manager/ai/... -run TestConvertTextToSQL -v passes. The new TestConvertTextToSQL/ValidQueryWithErrorAsValue case reproduces the reported bug: confirmed by stashing this change and re-running the same test, which fails with query is invalid on the old code and passes after this fix.
• gofmt -l on all changed files reports no issues.
• Could not run this repo's make test / make lint targets to completion: the local machine's disk is almost entirely full (roughly 300-900Mi free on a 228Gi volume), and pkg/core/manager/ai transitively depends on k8s.io/kubernetes/pkg/apiserver through pkg/kubernetes/registry, which this disk cannot fully compile with the repo's gcflags. This is a pre-existing local environment constraint unrelated to this change, not a failure of the change itself — the narrower go build/go test commands above did complete successfully against the real compiled package. Happy to address anything the full CI run surfaces.

Which issue(s) this PR fixes:

Fixes #735

[github-actions]

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[pujitha24]

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I have read the CLA Document and I hereby sign the CLA

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[pujitha24]

recheck

[pujitha24]

recheck

[pujitha24]

This is rebased and the checks are green; if anything would make it easier to review, I am happy to adjust it.

[elliotxx]

Nice catch, thanks for fixing this! 🙌 LGTM. CLA and license checks are green; the Check and Constraints workflows were waiting for approval, so I've kicked them off. Will merge once they pass.