fix: reject maxTrafficWeight below 1

[semx]

maxTrafficWeight is used as a divisor, but nothing stops it from being zero.

The problem

The CRD puts no bound on the field:

maxTrafficWeight:
  description: MaxTrafficWeight The total weight of traffic. If unspecified, it defaults to 100
  format: int32
  type: integer

Validation checks that each setWeight is between 0 and maxTrafficWeight, but never checks maxTrafficWeight itself. With maxTrafficWeight: 0 a setWeight: 0 step still passes, and checkReplicasAvailable divides by it:

desiredReplicas := (desiredWeight * totalReplicas) / weightutil.MaxTrafficWeight(c.rollout)

which panics:

panic: runtime error: integer divide by zero
    github.com/argoproj/argo-rollouts/rollout.(*rolloutContext).checkReplicasAvailable

The panic is recovered by the workqueue, but the reconcile for that rollout fails each time it is retried.

The fix

Reject values below 1 during validation, next to the existing maxTrafficWeight check, so the user gets an InvalidSpec condition explaining the problem. getRolloutValidationErrors runs before the reconcile touches the traffic weights, so the division is no longer reachable with a zero total weight.

Negative values are rejected too — they produce negative replica counts rather than a panic.

Test

Added a case to TestValidateRolloutStrategyCanary covering 0 and -1. It fails without the check and passes with it.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Published E2E Test Results

  4 files    4 suites   4h 3m 8s ⏱️
123 tests 111 ✅  7 💤  5 ❌
508 runs  465 ✅ 28 💤 15 ❌

For more details on these failures, see this check.

Results for commit b46c354.

[github-actions]

Published Unit Test Results

2 580 tests   2 580 ✅  3m 26s ⏱️
  130 suites      0 💤
    1 files        0 ❌

Results for commit b46c354.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.06%. Comparing base (d1dd467) to head (b46c354).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4924      +/-   ##
==========================================
- Coverage   85.12%   85.06%   -0.06%     
==========================================
  Files         166      166              
  Lines       19238    19240       +2     
==========================================
- Hits        16376    16367       -9     
- Misses       2019     2024       +5     
- Partials      843      849       +6     

Flag	Coverage Δ
e2e	52.52% <0.00%> (+0.01%)	⬆️
unit-tests	81.56% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.