fix(iaas): remove routing table from state when it no longer exists

[devpie]

Description

fixes #1749

When a stackit_network_area no longer exists (the area or its regional configuration was deleted outside of Terraform), the IaaS API answers HTTP 404 for the routing tables and routes that belonged to it (verified against the API, see #1749). Read() of stackit_routing_table turned that 404 into an error diagnostic: it called utils.LogError and then RemoveResource, and Terraform keeps the prior state when a read reports an error. So every refresh of the stale resource failed, and a removed block with lifecycle { destroy = true } failed with it. The same code removed the resource from state after any API error, which was equally ineffective for the same reason.

Changes:

• stackit_routing_table.Read() removes the resource from state on HTTP 404 without an error diagnostic and reports every other error while keeping the state — the pattern stackit_routing_table_route.Read() already uses. The error summary follows the other IaaS resources (Error reading routing table).
• Resource-level test for Read() against a mocked API: 200 (state compared field by field), 404, 403, 500 and a transport error. It fails on main for the 404, 403 and 500 cases.
• Schema descriptions and generated docs of both resources state that the platform deletes routing tables and routes together with their stackit_network_area_region, and how the provider reacts.

Verification against the real API (provider v0.114.0 vs. this build, OpenTofu 1.12.6; details in #1749): with the network area deleted, tofu plan -refresh-only fails on v0.114.0 with Error: Reading routing table and succeeds with this build, reporting routing table and route as deleted; tofu destroy -refresh=false succeeds for the already deleted routing table and route.

Not part of this PR: stackit_network_area_region and stackit_network_area_route have related gaps, see #1751.

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory) — not needed, no attribute changed
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here) — not changed; the existing routing table acceptance tests cover the normal lifecycle
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

🤖 Generated with Claude Code

[SerseusWasTaken]

Hello @devpie, thank you for the contribution.

I have created a internal tracking issue and we will review your contribution soon.

Best regards
Jonas from the STACKIT Developer Tools team