DHCP6: Mirror the prior DHCP rapid commit fix to DHCPv6

[rsmarples]

Fixes #690

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: c847fac7-b873-40f6-b6c4-36eff12b902e

📥 Commits

Reviewing files that changed from the base of the PR and between 0406dd8 and 9151877.

📒 Files selected for processing (1)

• src/dhcp6.c

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Walkthrough

The DHCPv6 client now uses the Rapid Commit “requested” policy check for message sizing, option emission, and discovery reply acceptance.

Changes

DHCPv6 Rapid Commit handling

Layer / File(s)	Summary
Rapid Commit option construction src/dhcp6.c	Message sizing and Rapid Commit option emission now occur only when Rapid Commit is requested.
Rapid Commit reply validation src/dhcp6.c	Discovery accepts a Rapid Commit reply only when Rapid Commit is requested.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Possibly related issues

• NetworkConfiguration/dhcpcd issue 692 — It also concerns Rapid Commit policy checks and DHCPv6 reply acceptance.

Possibly related PRs

• NetworkConfiguration/dhcpcd#694 — It changes DHCPv6 Rapid Commit policy checks in src/dhcp6.c from “allowed” to “requested.”

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The change addresses DHCPv6 rapid commit behavior and prefix delegation, but the linked issue also requires restoring routable IPv4 connectivity, which is not covered here.	Add or link code changes that address the linked issue's IPv4 connectivity regression, or update the issue scope if the IPv4 fix is intentionally excluded.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the DHCPv6 rapid commit fix and matches the primary code change.
Description check	✅ Passed	The description links the pull request to issue #690, which directly relates to the reported DHCP connectivity regression.
Out of Scope Changes check	✅ Passed	The three-line change is limited to DHCPv6 rapid commit sizing, emission, and reply validation, all of which support the linked issue.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch dhcp6-rapidcommit

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