v0.3.0 — real-world test surfaces 9 fixes + nested resource layout

Nine fixes surfaced by generating a working SDK from the real
openai-openapi spec (162 paths, 983 schemas) — each oracle-verified
against the actual Stainless-generated openai-python SDK. Plus the
nested resource directory layout that matches openai-python's
structure (resources/chat/completions/messages.py etc.) — drop-in
import paths.

Fixed

• Silent leaf-name collision (correctness bug). With the previous
  flat resources/*.py layout, two subresources sharing a leaf name
  (e.g. chat.completions.messages AND threads.messages) wrote to
  the same file; last-wins meant c.chat.completions.messages.list()
  would silently call /threads/{thread_id}/messages (wrong
  endpoint). Now: nested directories matching openai-python
  (resources/chat/completions/messages.py,
  resources/threads/messages.py). Cross-cutting imports inside
  resource files switched from .. to absolute <pkg>._core.X so the
  same template works at any depth. New regression test
  (test_subresource_with_shared_leaf_name_calls_right_endpoint).
• Discriminated union with an Any variant (real openai spec
  pattern: oneOf: [{type:object, properties:{type,...}}, {}] where
  the empty branch lowers to AnyType). Pydantic rejects object as
  a discriminated-union variant at import time. Now: drop the AnyType
  variants from a discriminated union before applying the
  discriminator.
• Field-name shadows Python builtin used as a type annotation.
  Real example: ConversationResource has
  object: Literal['conversation'] AND metadata: object. Mypy's
  class-scope name resolution finds the field, not the builtin →
  error. Now: when a field is named after a Python builtin AND that
  name is used as a type annotation by another field, the
  builtin-named field is reordered to come AFTER any such use (within
  the same required-vs-optional cohort, so pydantic field ordering
  stays valid). Oracle: openai-python's Conversation lays out
  id, metadata, object, ... for exactly this reason.
• allOf merger was too strict at spec scale. Rejected
  real-world allOf branches that differed only in (a) cosmetic
  fields (description / title / example), (b) $ref vs inlined-same-
  schema, (c) enum membership where Stainless takes the union
  (oracle: openai-python emits Literal["completed", "incomplete", "failed", "in_progress"] — the union of two branches), or (d) a
  T-or-null vs T lattice (oracle: openai-python emits
  Optional[int] when one branch is nullable and the other isn't).
  Now: cosmetic-strip + one-hop $ref resolve + enum-union +
  nullable-lattice; remaining structural divergence falls through to
  lenient last-wins. Honest trade-off: strict raise was right for
  in-repo fixtures, wrong at real-spec scale.
• cast_to: type | None was too narrow. The
  Annotated[Union[V0, V1], Field(discriminator="task")] shape for a
  discriminated-union response (real example: openai
  audio.transcriptions.create) failed mypy AND silently fell
  through to raw dict at runtime. Now: signature widened to Any;
  _process_response_data switched to a TypeAdapter-first pattern
  (try pydantic.TypeAdapter(cast_to), fall back to raw data only on
  PydanticSchemaGenerationError) — passes mypy, validates discriminated
  unions, and bytes / None short-circuits still work.

Added

• Real-world test: examples/openai/ — the public Stainless-
  processed openai-openapi spec (162 paths, 983 schemas, OpenAPI
  3.1) + a hand-written stainless.yml covering ~22 resources (chat,
  embeddings, files, audio, fine_tuning, models, moderations, images,
  batches, assistants, uploads, vector_stores, conversations,
  threads, …). Generates a mypy-clean 172-file SDK that exercises
  every capability stainful supports: streaming, JSON, multipart,
  binary download, raw binary upload, cursor pagination,
  discriminated-union responses, nested subresources, webhook unwrap.
  Generated SDK is gitignored; spec committed (content-pinned via the
  Stainless CDN SHA in the source URL).
• Pydantic-reserved field-name mangling. Field names that clash
  with pydantic.BaseModel attributes (schema, copy, dict,
  json, model_dump, ...) are now mangled to <name>_ and aliased
  via Field(alias="<wire>") — same pattern as the existing
  Python-keyword mangle. Oracle: openai-python emits
  schema_: Optional[Dict[str, object]] = FieldInfo(alias="schema", default=None).
• Compound brand name override. openai → OpenAI (not
  Openai), openapi → OpenAPI, anthropic → Anthropic,
  cloudflare → Cloudflare. Hardcoded table (no risky auto-split
  on initialism suffixes — would over-match real words like chai,
  tai). Stainless's custom_casings block is the long-term answer
  (v1.1 backlog).
• Adoption polish. New
  docs/migrating-from-stainless.md:
  step-by-step migration guide for Stainless customers (after the
  Anthropic acquisition wind-down), exhaustive symbol-compat table,
  honest gap list with workarounds. README leads with a migration
  callout; OpenAI proof point in the Status section. Quickstart
  trimmed to the essential install + generate; contributor commands
  moved out of the user flow.

Verified

• 84 tests green (was 77 at v0.2.0; 7 new regression tests for
  the cross-spec bugs).
• mypy 0 on 250 combined source files (172 generated openai SDK +
  92 dogfood + 9 emitter).
• Drop-in import parity: from openai.resources.chat.completions import CompletionsResource resolves against stainful's output.
• No regressions on the OneBusAway dogfood: 29/29 of Stainless's
  own test files still import unchanged.