feat(server): TypeVar-bound ADCPHandler for typed ToolContext subclasses (#223)

[bokelley]

Summary

Closes #223 (and roadmap PR-Q). Multi-tenant agents routinely subclass `ToolContext` to carry typed tenant/adapter/testing fields. Before this PR those fields needed a `cast` or `isinstance` at every handler method. Now:

```python
from dataclasses import dataclass
from adcp.server import ADCPHandler, ToolContext

@DataClass
class MyContext(ToolContext):
adapter: MyPlatformAdapter

class MyAgent(ADCPHandler[MyContext]):
async def get_products(self, params, context: MyContext | None = None):
if context is not None:
adapter = context.adapter # typed, no cast
```

• New `TContext = TypeVar("TContext", bound="ToolContext")` exported from `adcp.server`.
• `ADCPHandler` now `Generic[TContext]`; all 57 method signatures take `context: TContext | None`.
• Protocol handlers (`BrandHandler`, `ComplianceHandler`, `ContentStandardsHandler`, `GovernanceHandler`, `SponsoredIntelligenceHandler`, `TmpHandler`) propagate the TypeVar so downstream can write `class MyBrand(BrandHandler[MyContext])`.
• Internal SDK call sites use `ADCPHandler[Any]` where the SDK doesn't care about the subclass.

Backward compatibility

`class MyAgent(ADCPHandler)` without a TypeVar argument still works at runtime — Python allows unparameterised Generic inheritance. Every existing adopter keeps working without edits.

Test plan

• `pytest tests/` — 1544 passed, 15 skipped (+9 from this PR)
• `ruff check src/ tests/` — clean
• `mypy src/adcp/` — 0 errors
• New `tests/test_handler_typevar.py` covers:
  • unparameterised subclass still works (backward compat)
  • `ADCPHandler[MyContext]` / `BrandHandler[MyContext]` construct cleanly
  • handler methods receive the typed subclass at dispatch (`isinstance(context, MyContext)` is true at runtime)
  • A2A executor dispatches a typed handler without issue
  • `TContext.bound` is `ToolContext`
  • 57 method signatures preserve `(self, params, context)` positioning

Follow-ups (not in this PR)

• PEP 696 TypeVar default (`TContext = TypeVar("TContext", bound=ToolContext, default=ToolContext)`) requires Python 3.13+. We support 3.10+, so no default — unparameterised `ADCPHandler` resolves to `ADCPHandler[Any]` in mypy. Revisit when the floor moves.
• Protocol-class version of `SkillMiddleware` / `ContextFactory` for better IDE hover signatures — flagged by feat(server): per-skill middleware hook in ADCPAgentExecutor (#226) #233's review.

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