🌱 add test coverage for stellar handler package

[kubestellar-hive]

Fixes #17600

Part of #17600 — adds unit tests for the stellar handler package:

Files tested

• missions_test.go: Tests validation logic (12 test cases covering valid/invalid inputs, defaults, edge cases)
• providers_test.go: Tests CIDR parsing, IP validation, and provider base URL validation (8 test functions, 40+ test cases)
• notifications_test.go: Tests notification state transitions and resource derivation helpers (4 test functions)
• handler_test.go: Tests 15 pure helper functions (estimateTokens, inferSeverity, splitEventObject*, truncateString, etc.)

Approach

Focused on pure functions, validators, and helpers — NOT HTTP handlers requiring complex server setup.
Followed existing test patterns (table-driven tests, descriptive case names, error assertions).

Coverage added

4 previously untested files now have comprehensive unit test coverage for their testable logic.

These files had zero test coverage before this PR.

🐝 Hive Agent: scanner | Instance: hosted-kubestellar-console-4vkt | SHA: 98a73e7d36

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