refactor: Stage 2: Adopt new way for providers to register, test with 4 providers

[TomOnTime]

Provider implementations currently register roles, constructors, credential metadata, and capabilities separately. This adds providers.Register[T](name, Definition{...}) to collect that information in one definition and derive initialization, auditing, roles, and zone operations from the implementation's interfaces.

• Migrate NONE, BIND, DNSimple, Route 53, and Cloudflare (CLOUDFLAREAPI).
• Allocate a fresh instance per factory call, enforce CreateOptions.RequestedRole, and preserve Route 53's registrar-only region check and conversion observers.
• Resolve canonical names and aliases through one map of shared definition pointers, documented as read-only. Enumerate canonical definitions once in sorted order.
• Derive DocsURL from the lowercase canonical type name. Preserve explicit legacy-path overrides, reject overrides equal to the default during registration, and remove redundant URLs from the pilot definitions. Test defaults, overrides, alias lookups, and rejection independently of later stages.
• Keep compatibility registries for existing consumers and unmigrated providers, preserving legacy feature notes and record-type validation.
• Inline single-use maintainer constants, provider names, and feature maps in the migrated registrations; retain provider-name constants shared with custom record registration.
• Cover registration errors, alias collisions, pointer identity, role enforcement, independent account caches, credential-free auditing, and provider initialization with local mocks.
• Give the JavaScript syntax-test mock its own GetZoneRecords method and remove its embedding of NONE, which now implements only the registrar role.

This is Stage 2 of the provider API rollout. Stage 1 (#4955) has merged, and this branch includes the current syntax-test compatibility adjustment. Remaining provider migrations, consumer/alias consolidation, and SupportedTypes validation will follow in separate PRs. RecordAuditingProvider remains a temporary migration interface; AuditRecords will become part of models.DNSProvider at the end of Stage 3.

Validation at fdeac2b3:

• go test ./...
• bin/generate-all.sh: formatting, generation, module tidy, go fix ./..., repository-wide golangci-lint (0 issues), and staticcheck all pass; no additional tracked changes.
• git diff --check

Route 53 and Cloudflare regression tests use local mocks; no live provider integration tests or DNS mutations were performed.