fix: remove @runtime_checkable from protocols; use hasattr checks in validation

[bkrabach]

Problem

On Python 3.11, isinstance(tool, Tool) returns False for every conformant tool. The root cause is:

1. Tool (and all other core protocols) are decorated with @runtime_checkable
2. A Tool.__protocol_attrs__ override was added (line 174 of interfaces.py) to exclude input_schema from isinstance checks for backward compatibility
3. __protocol_attrs__ is a Python 3.12-only feature. On Python 3.11, _get_protocol_attrs() walks cls.__dict__ rather than reading that attribute — so __protocol_attrs__ leaks as a spurious required member, and any tool that doesn't expose it as an instance attribute fails the isinstance check

This caused test_tool_without_input_schema_satisfies_isinstance to fail on Python 3.11 before this fix.

The Fix

interfaces.py

• Remove @runtime_checkable from all 6 protocols: Orchestrator, Provider, Tool, ContextManager, HookHandler, ApprovalProvider
• Remove the Tool.__protocol_attrs__ hack
• Remove the now-unused runtime_checkable import

The Protocol classes themselves are kept intact — they remain importable contract documentation with IDE hover support and static type checking via pyright/mypy. @runtime_checkable was only serving the validation/ dev tooling, and it required version-specific workarounds to work.

validation/ (tool, provider, orchestrator, hook, context)

• Replace all isinstance(obj, Protocol) checks with structural hasattr() helper functions:
  • _implements_tool_interface(obj) — checks name, description, execute
  • _implements_provider_interface(obj) — checks name, get_info, list_models, complete, parse_tool_calls
  • _implements_orchestrator_interface(obj) — checks execute
  • _implements_hook_handler_interface(obj) — checks callable()
  • _implements_context_manager_interface(obj) — checks all 5 required methods
• Improved error messages now name the missing member(s) instead of just returning a boolean
• Updated module docstrings

This matches what the kernel runtime already does: session.py, coordinator.py, and amplifier-foundation all use hasattr() duck typing — never isinstance() against Protocol types.

tests/test_interfaces.py

• Replace the now-failing test_tool_without_input_schema_satisfies_isinstance (which demonstrated the Python 3.11 bug) with test_tool_without_input_schema_satisfies_structural_check using hasattr()
• Add test_tool_is_not_runtime_checkable — regression guard
• Add test_orchestrator_is_not_runtime_checkable

tests/test_validation.py

• Update message string assertions to match updated error message wording ("interface" instead of "protocol")

Verification

• Python 3.11.14 (venv): 566 passed, 1 skipped ✅
• cargo test -p amplifier-core: 19 doc-tests passed ✅

Before / After

Scenario	Before	After
isinstance(tool, Tool) on Python 3.11	❌ Returns False (due to __protocol_attrs__ leak)	N/A — no longer used in validation
isinstance(tool, Tool) on Python 3.12	✅ Returns True	N/A
Validation detects conformant tool	✅ 3.12 only	✅ Python 3.11, 3.12, 3.13
Validation detects non-conformant tool	✅	✅ (with better error messages)
Runtime (session, coordinator) behavior	Unchanged — always used duck typing	Unchanged