fix(ai): handle filter-only searches and JSON-safe tool errors

[vitorhugo-dotnet]

Closes #85

What changed

• allow searchApplications without a free-text query when at least one meaningful structured filter is present
• reject completely unfiltered searches
• skip FULLTEXT/LIKE predicates for filter-only searches and preserve deterministic ordering
• mark nullable search parameters as optional in the Spring AI tool schema
• make Spring AI tool execution errors JSON-safe at the exception-processor boundary for Gemini

Tests

• service regression for null/blank free-text with structured filters
• repository SQL regression proving filter-only searches do not use MATCH/LIKE
• contextual CWI follow-up regression through AssistantTools
• application-context regression proving tool errors are valid JSON
• mvn -B -ntp clean verify ✅ (GitHub Actions)

TDD evidence

• RED: regression commit reproduced 4 failures + 1 error across the acceptance scenarios
• GREEN: workflow run 35042918961, job 104626448677 completed successfully

[github-actions]

Qodana for JVM

3 new problems were found

Inspection name	Severity	Problems
Nullability and data flow problems	🔶 Warning	1
Mismatched query and update of 'StringBuilder'	🔶 Warning	1
Simplifiable conditional expression	🔶 Warning	1

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