fix(api): validate timeout is positive in PostSandboxesSandboxIDConnect

[chill-czar]

Closes #3579

Summary

• Enforce if body.Timeout <= 0 validation in PostSandboxesSandboxIDConnect (POST /sandboxes/{id}/connect), returning 400 Bad Request ("Timeout must be greater than 0") when timeout is non-positive.
• Bring PostSandboxesSandboxIDConnect in alignment with PostSandboxes, PostSandboxesSandboxIDResume, and PostSandboxesSandboxIDFork.
• Add unit test coverage in packages/api/internal/handlers/sandbox_timeout_validation_test.go verifying zero and negative timeout values are rejected early.

Why

In packages/api/internal/handlers/sandbox_connect.go, body.Timeout was only validated against the maximum length limit (MaxLengthHours). Supplying 0 or negative values resulted in a negative/zero sandbox lifetime passed to startSandbox, which immediately terminated the connected sandbox due to the expiration timestamp being in the past.

Diff Overview

 	body, err := ginutils.ParseBody[api.PostSandboxesSandboxIDConnectJSONRequestBody](ctx, c)
 	if err != nil {
 		a.sendAPIStoreError(c, http.StatusBadRequest, fmt.Sprintf("Error when parsing request: %s", err))
 
 		telemetry.ReportCriticalError(ctx, "error when parsing request", err)
 
 		return
 	}
 
+	if body.Timeout <= 0 {
+		a.sendAPIStoreError(c, http.StatusBadRequest, "Timeout must be greater than 0")
+
+		return
+	}
+
 	timeout := time.Duration(body.Timeout) * time.Second
 	if timeout > time.Duration(teamInfo.Limits.MaxLengthHours)*time.Hour {
 		a.sendAPIStoreError(c, http.StatusBadRequest, fmt.Sprintf("Timeout cannot be greater than %d hours", teamInfo.Limits.MaxLengthHours))

Test Plan

• Unit test: go test -v ./packages/api/internal/handlers -run TestSandboxConnect_RejectsNonPositiveTimeout
• Package test suite passes: go test -v ./packages/api/internal/handlers/...
• Linter & Formatter pass: make fmt and make lint
• Confirmed timeout: 0 and timeout: -1 return 400 Bad Request ("Timeout must be greater than 0")

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