revert(preview): remove opt-in v3.1 preview surface (#747) (#755)

Reverts the v3.1.0-beta.1 client surface from PR #747. The Python SDK
will follow the TypeScript pattern instead: stay clean on 5.x against
AdCP 3.0.x, cut a new major (6.0) when AdCP 3.1 stable lands and run
codegen against the clean spec. Hand-rolled 3.1 types in preview.py
would diverge from the TS approach and accumulate drift as 3.x evolves;
two-surface confusion (``adcp.types`` vs ``adcp.preview``) is exactly
what pre-bid / sales-agent adopters can't afford.

PR #747 was never released to PyPI — v5.6.0 was tagged before #747
merged, so the preview surface only ever existed on ``main`` between
#747 (14:18 UTC) and this revert (~17:30 UTC). No downstream wheel
ever carried it.

Removes:
- src/adcp/preview.py (1079 lines)
- tests/test_preview.py, tests/test_preview_e2e.py (914 lines)
- ADCPClient.preview property and catalog_change_feed_client() method
- README v3.1 preview-surface quick-start section
- schemas/cache/3.1.0-beta.1/ (vendored bundle, 40MB, 661 files)

The bundle was kept in the initial revert PR as 6.0-prep convenience,
but expert review caught a real DX trap: with the bundle on disk but
no v3.1 Pydantic types, ``schema_loader._ensure_state`` would route
v3.1 wire responses to JSON-Schema validation against 3.1 schemas
that then parse through v3.0 types. Silent validate-against-different-
types is worse than no validation. Dropping the bundle makes the SDK
treat v3.1 like any unknown future version: skip validation, parse
through v3.0 with extras preserved (loud failure when an unsupported
attribute is accessed). Re-sync at 6.0 prep with one command.

Keeps:
- src/adcp/validation/version.py wire-format prerelease resolution
  (pure infrastructure; resolves ``3.1-beta.1`` -> ``3.1.0-beta.1``
  bundle key, used generally for any prerelease normalization)
- tests/test_validation_version.py
- scripts/sync_schemas.py multi-bundle machinery
  (PREVIEW_VERSIONS = () so no extra bundles sync; flipping it back
  on is one line when 6.0 prep starts)
- README ``## AdCP version support`` tombstone pointing at 6.0 and
  #741

Codegen workarounds I built while trying to put 3.1.0-beta.1 through
codegen are captured in .context/codegen-prep/ for 6.0 prep (not in
this PR — they're prep notes, not production code). Companion upstream
issues filed at:
- koxudaxi/datamodel-code-generator#3158 (allOf+oneOf drops oneOf)
- koxudaxi/datamodel-code-generator#3159 (nested $ref via allOf)
- koxudaxi/datamodel-code-generator#3160 (fabricated Literal['reuse'])
- adcontextprotocol/adcp#4859 (bundled mega-schema inlining)

Expert reviews (code-reviewer-deep, security-reviewer-deep, adtech-
product-expert, dx-expert) recommended dropping the bundle and adding
the tombstone; both addressed here.

Test plan:
- ruff check src/ -> clean
- mypy src/adcp/ -> clean (806 source files)
- pytest tests/ -> 4883 passed, 0 failed, 26 skipped, 1 xfailed

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bokelley

README.md

@@ -207,54 +207,15 @@ async with ADCPMultiAgentClient(
 # Connections automatically cleaned up here
 ```
 
-## Quick Start: v3.1 catalog-sync preview (opt-in)
-
-The `adcp.preview` module exposes the three v3.1 catalog-sync proposals
-([adcontextprotocol/adcp#4761](https://github.com/adcontextprotocol/adcp/issues/4761),
-[#4762](https://github.com/adcontextprotocol/adcp/issues/4762),
-[#4763](https://github.com/adcontextprotocol/adcp/issues/4763))
-as opt-in client features. The SDK's default AdCP pin stays on the
-current stable line — importing `adcp.preview` does not change wire
-behaviour for any other call, and adopters who never import it pay
-nothing. Use it against v3.1.x agents during the beta.
-
-```python
-from adcp import ADCPClient, AgentConfig, Protocol
-from adcp.preview import CatalogVersionCache, GetProductsRequest
-
-cache = CatalogVersionCache()
-
-async with ADCPClient(AgentConfig(
-    id="seller",
-    agent_uri="https://seller.example",
-    protocol=Protocol.MCP,
-)) as client:
-    request = GetProductsRequest(buying_mode="wholesale")
-
-    # First call: full catalog payload + a catalog_version token cached.
-    result = await client.preview.get_products(request, cache)
-    # Second call: SDK auto-attaches if_catalog_version. If the seller's
-    # catalog is unchanged, the response is `unchanged: true` and the
-    # cached payload is returned without re-fetching the inventory.
-    result = await client.preview.get_products(request, cache)
-
-    # Capability probe + change-feed poll for near-real-time mirroring.
-    async with client.catalog_change_feed_client() as feed:
-        page = await feed.poll(cursor=None)
-        for event in page.events:
-            print(event.event_type, event.entity_id)
-```
-
-`adcp.preview` re-exports `GetProductsRequest` / `GetSignalsRequest`
-(plus matching response types) as subclasses of the v3.0 generated
-models — they accept the same fields plus the v3.1 conditional-fetch
-tokens (`if_catalog_version`, `if_pricing_version`) and `discovery_mode`
-for `get_signals`. Wholesale enumeration: `GetSignalsRequest(discovery_mode="wholesale")`
-or `GetProductsRequest(buying_mode="wholesale")`.
-
-For details on the cache scoping rules, two-layer (`public` / `account`)
-safety invariant, and webhook subscription endpoint, see the inline docs
-on [`src/adcp/preview.py`](src/adcp/preview.py).
+## AdCP version support
+
+The 5.x line targets AdCP 3.0 stable. v3.1 support lands in SDK 6.0 against
+the 3.1 stable spec — there is no opt-in preview surface in 5.x. If you talk
+to a v3.1+ agent from 5.x, the SDK parses the response through v3.0 types
+(unknown fields are preserved on the model but not surfaced as typed
+attributes) and schema validation is skipped for that version. Track
+[#741](https://github.com/adcontextprotocol/adcp-client-python/issues/741)
+for 6.0 progress.
 
 ## Documentation