[User Story] Pin vultr provider to v2.29.0 (skip v2.30.0+ ReadResource regression)

[noahwhite]

Story Summary

As a platform engineer, I want the Vultr OpenTofu provider pinned to v2.29.0, so that plan CI doesn't crash on vultr_block_storage refresh due to a regression introduced in v2.30.0 and persisting through v2.31.0.

---

✅ Acceptance Criteria

• vultr/vultr provider constraint is = 2.29.0 in all four locations: envs/dev/main.tofu, modules/vultr/block_storage/main.tofu, modules/vultr/firewall/main.tofu, modules/vultr/instance/main.tofu
• .terraform.lock.hcl regenerated with 2.29.0 hashes
• Renovate config updated with a packageRule that excludes vultr/vultr versions > 2.29.0 until the regression is resolved upstream
• tofu fmt -check -recursive opentofu/ passes
• pr-tofu-plan-develop CI is green (no Plugin did not respond crash)
• Renovate PR chore(deps): update terraform vultr to v2.31.0 #368 (v2.31.0 bump) is closed

---

📝 Additional Context

• Regression boundary: vultr_block_storage ReadResource panic (Plugin did not respond) was confirmed on v2.30.0, v2.30.1, and v2.31.0. v2.29.0 is the highest clean version.
• Current version before fix: = 2.28.1 (pinned in GHO-109 / PR feat(GHO-109): upgrade Vultr provider to v2.28.1, remove block storage attachment workaround #318)
• Target version: = 2.29.0
• Module state: The null_resource attach workaround was removed in GHO-109 and does not come back with this change; we continue to rely on native attached_to_instance support.

Known side effect (separate follow-up): Upgrading from v2.28.1 → v2.29.0 causes vultr_firewall_rule resources to plan -/+ destroy and then create replacement because the provider changed how the source attribute is represented in state. Our module already uses subnet/subnet_size so configuration is correct; this is a one-time state-migration artifact. Will be handled in a follow-up (accept recreation with brief origin-firewall gap, or state rm+import to avoid).

---

📦 Definition of Ready

• Acceptance criteria defined
• No unresolved external dependencies
• Story is estimated
• Team has necessary skills and access
• Priority is clear
• Business value understood

---

✅ Definition of Done

• All acceptance criteria met
• Unit/integration tests written & passing
• Peer-reviewed
• Docs updated (if applicable)
• Verified in staging (if needed)
• No critical bugs/regressions

[linear]

GHO-131