refactor(sdk): harness profiles for provider/model config

[Mason Daugherty (mdrxy)]

Calling refactor since this has no user-facing impact.

Consolidate provider-specific logic (hardcoded if model.startswith(...) branches, per-provider kwargs, prompt tweaks) into a declarative, internal-only HarnessProfile registry. No user-facing API changes.

Each provider or exact model spec can register a profile that customizes init kwargs, system prompts, tool descriptions, tool exclusions, and middleware — eliminating scattered conditionals. Subagents resolve their own profile from their model, so mixed-provider setups get correct behavior automatically.

Changes

• Add private HarnessProfile dataclass and a registry with exact-spec -> provider-prefix -> empty-default lookup and merge semantics
  • (dict fields merge, callables chain, middleware deduplicates by type, excluded_tools unions)
• Wire profiles into create_deep_agent: apply tool_description_overrides to user tools, FilesystemMiddleware, and SubAgentMiddleware; assemble system prompt from base_system_prompt + system_prompt_suffix; inject extra_middleware into all three middleware stacks (main, general-purpose subagent, per-subagent)
• Add internal _ToolExclusionMiddleware that strips excluded_tools from request.tools before the model sees them — placed late in the stack so it catches both user-supplied and middleware-injected tools