[feat] add support for reserved IPv4 range in NodeBalancer backend allocation

[vsilva-akamai]

Summary

• Adds --nodebalancer-backend-ipv4-reserved-range and the matching Helm value for reserving the highest /30 in a configured NodeBalancer backend subnet.
• Allocates the first available /30 below the reserved range by reading the subnet's existing NodeBalancer assignments.
• Rejects invalid reservations, existing assignments that use the reserved range, and service-provided backend ranges that overlap the reservation.
• Adds unit coverage for allocation, validation, VPC option construction, and Helm rendering.

Rationale

Reserving the highest /30 prevents CCM-managed NodeBalancer backend allocation from consuming an address range that must remain available for networking.

Validation

• go test ./cloud/linode -count=1
• make helm-template

General

• Have you removed all sensitive information, including but not limited to access keys and passwords?
• Have you checked to ensure there aren't other open or closed Pull Requests for the same bug/feature/question?

Pull Request Guidelines

1. Does your submission pass tests?
2. Have you added tests?
3. Are you addressing a single feature in this PR?
4. Are your commits atomic, addressing one change per commit?
5. Are you following the conventions of the language?
6. Have you saved your large formatting changes for a different PR, so we can focus on your work?
7. Have you explained your rationale for why this feature is needed?
8. Have you linked your PR to an open issue

[codecov]

Codecov Report

❌ Patch coverage is 60.15038% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.17%. Comparing base (34ce86e) to head (2871942).

Files with missing lines	Patch %	Lines
cloud/linode/nodebalancer_backend_range.go	59.77%	18 Missing and 17 partials ⚠️
cloud/linode/loadbalancers.go	70.00%	6 Missing and 6 partials ⚠️
cloud/linode/cloud.go	0.00%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #632      +/-   ##
==========================================
- Coverage   75.93%   75.17%   -0.77%     
==========================================
  Files          18       19       +1     
  Lines        2813     2940     +127     
==========================================
+ Hits         2136     2210      +74     
- Misses        464      493      +29     
- Partials      213      237      +24     

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[AshleyDumaine]

Can you document the new feature on docs/configuration/environment.md and docs/configuration/loadbalancer.md as well?
I think we'll also want a new chainsaw e2e test for this.

[vsilva-akamai]

Can you document the new feature on docs/configuration/environment.md and docs/configuration/loadbalancer.md as well? I think we'll also want a new chainsaw e2e test for this.

I updated the docs as requested, and now I'm working on getting the Chainsaw tests implemented and working.

[vsilva-akamai]

Can you document the new feature on docs/configuration/environment.md and docs/configuration/loadbalancer.md as well? I think we'll also want a new chainsaw e2e test for this.

Tests implemented.

outdated