fix(api): add nil guard in volume mount response conversion

[chill-czar]

Closes #3450

Summary

• Add if item == nil { continue } guard in convertFromDBMountsToAPIMounts within packages/api/internal/handlers/sandboxes_list.go.
• Add TestConvertFromDBMountsToAPIMounts unit tests in packages/api/internal/handlers/volume_util_test.go asserting nil items are skipped safely without panicking.

Why

When converting database volume mount configs to API response models, convertFromDBMountsToAPIMounts iterated over the slice items directly. If a nil pointer existed in the slice (e.g. from JSONB unmarshaling edge cases), dereferencing item.Name or item.Path triggered a nil pointer panic, crashing the active request handler goroutine for GET /sandboxes/{id} or GET /v2/sandboxes. Adding an explicit nil check guarantees handler safety.

Test Plan

• Unit tests pass: go test -race -v -run TestConvertFromDBMountsToAPIMounts ./packages/api/internal/handlers/...
• Verified nil slice element handling without panics
• Formatted code cleanly: go fmt ./...
• No regressions in volume utility handler tests

/cc @jakubno @dobrac @ValentaTomas @arkamar @tvi

[chill-czar]

@codex

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

Reviewed commit: e9031e3557

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