feat(networking): Neutron QoS family (policy + bandwidth-limit / DSCP / minimum-bandwidth rules)

[PF9-pushkar]

Summary

Adds the Neutron QoS resource family (Phase 3), ported from terraform-provider-openstack v3.4.0 and adapted to the terraform-plugin-framework:

Resource	Purpose
pcd_networking_qos_policy	QoS policy (shared, is_default, tags)
pcd_networking_qos_bandwidth_limit_rule	max_kbps / max_burst_kbps / direction
pcd_networking_qos_dscp_marking_rule	dscp_mark
pcd_networking_qos_minimum_bandwidth_rule	min_kbps / direction
pcd_networking_qos_policy (data source)	look up a policy by id or name

Design notes

• No new client/subcategory. The family lives in the existing internal/services/networking package and reuses configureClient, replaceTags, and NetworkV2Client. Docs land under the existing Networking subcategory.
• Rules are nested under a policy. qos_policy_id is Required + ForceNew; rules import via a composite <qos_policy_id>/<rule_id> ID (helper splitQoSRuleID). The policy imports by bare ID.
• Update asymmetry. Policy/rule UpdateOpts set pointer fields only when the planned value differs from state, so a boolean flipped to false is still sent (verified against gophercloud BuildRequestBody).

Adversarial review — two real bugs found and fixed

Three review passes (state-consistency, gophercloud API usage, import/data-source/registration) ran before this PR. Two confirmed findings, both fixed in this branch:

1. Wrong tags path (high). Policy tags were written to the policies attributes-tags collection; the QoS policy collection is qos/policies (/v2.0/qos/policies/{id}/tags). Tag writes would have 404'd against a real cloud. Fixed at both call sites; confirmed against upstream terraform-provider-openstack and gophercloud's resourcePath.
2. Discarded notFound after create/update (correctness). Create/Update ignored the readInto not-found bool; a read-after-write 404 would persist unknown computed values → "Provider produced inconsistent result after apply". Now a post-write 404 is surfaced as an error in all four resources.

The import/data-source/registration pass returned no findings.

Testing

• go build, go vet, gofmt, golangci-lint — all clean (0 issues).
• Unit tests pass; terraform fmt clean on examples.
• Full-tree acceptance test (TestAccNetworkingQoS_tree): policy + all three rule types + data source, an in-place rename/rate-change step, and policy import — not run live this session (CE lab credentials unavailable). QoS depends only on the Neutron qos extension (no compute/storage backend), so it should pass on the fresh CE lab; see DECISIONS.md.

Docs / examples

• Examples for every resource (with import.sh) and the data source.
• docs/ regenerated via make generate (not committed — generated on demand).
• CHANGELOG and DECISIONS updated.