refactor: introduce modelsdev.ID for provider-qualified model identity

[dgageot]

Fixes #2743.

Replaces bare string model identifiers with a strongly-typed
modelsdev.ID (provider + model name) on the API surface that
the issue called out as bug-prone, so the compiler refuses
unqualified model strings where a provider/model pair is
required.

What changes

New type — pkg/modelsdev/id.go:

• ID{Provider, Model} with NewID, ParseID, ParseIDOrZero,
  String, IsZero, IsValid.

API now takes/returns modelsdev.ID instead of string:

• modelsdev.Store.GetModel, runtime.ModelStore.
• provider.Provider.ID() and base.Config.ID(), plus all
  implementations (anthropic, bedrock, gemini, openai, oaistream,
  dmr, vertexai/modelgarden, rulebased, …).
• modelinfo.LoadCaps, modelinfo.IsClaude,
  modelinfo.LookupFamily.
• oaistream.ConvertMessages / ConvertMultiContent,
  bedrock.convertMessages, gemini.convertMessagesToGemini,
  the per-provider convertDocument helpers,
  bedrock.detectCachingSupport.
• rulebased.Client.LastSelectedModelID.
• Embedding/RAG: strategy.EmbeddingConfig.ModelID,
  strategy.VectorStoreConfig.ModelID, internal modelStore
  interface, calculateSemanticUsageCost.

Internal adapters:

• runtime.getAgentModelID / getEffectiveModelID return
  modelsdev.ID; agent-info events convert via .String() at
  the boundary.
• team.AgentInfo reads Provider/Model from the ID directly
  instead of splitting a string.
• runtime.strip_modalities parses hooks.Input.ModelID via
  modelsdev.ParseID.

Why

Several call sites used to pass a bare model name (e.g.
"claude-sonnet-4-6") where a provider/model pair was
expected. The compiler had no way to flag these mistakes and they
silently fell through to "model not found" lookups. Lifting the
identity to a dedicated type makes the contract explicit and
unforgeable.