v3.5.0: knowledge parameter and knowledge result models

Minor release. Adds support for the new knowledge parameter on
POST /v1/search and the knowledge result models that come back with it, and
closes two documented response fields the SDK had been dropping. Purely
additive — no breaking changes.

Added

• knowledge parameter on search() and search_async() — pass
  knowledge="core" to request knowledge results backed by licensed data
  providers such as encyclopedias, market-data firms, and reference publishers.
  They arrive in their own section at response.results.knowledge. "core" is
  the only value the API accepts, and anything else raises ValidationError
  locally rather than reaching the network, mirroring the server's 422.
• Knowledge enum — Knowledge.CORE, exported from youdotcom.models.
  Plain strings are accepted and normalized, so knowledge="core" and
  knowledge="CORE" both work.
• KnowledgeResult and KnowledgeAttribution models — a knowledge result
  carries type, title, and attribution, plus description and an optional
  as_of date for type: answer results, the only kind returned today. type
  is modeled as a plain str so an unrecognized kind parses instead of raising,
  since a new kind may populate a different set of fields. Attribution entries
  are credits rather than citations and carry no URL.
• Results.knowledge — new optional field on the search response container.
  Up to 25 results are returned, limited to those relevant to the query. When
  none are relevant the API omits the key entirely, so the field is None
  rather than an empty list and iterating needs an or [] guard. count caps
  the web and news sections, not knowledge.

Fixed

• AnswerSearchResult was dropping description and thumbnail_url — the
  answer spec defines both on results.web[] and the API returns them on every
  result, but the model did not declare them, so they were discarded at parse
  time. Both are now optional str fields. WebResult on the search endpoint
  already had them; the answer model was simply the narrower of the two.
• FinanceResearchSource was missing snippets — the finance-research spec
  defines it on output.sources[], and the sibling Source model on the
  Research API already declared it. Production was not returning the field at
  the time of this release, so nothing was being lost yet, but the model now
  matches the published contract instead of relying on a drift-checker
  suppression that could not have noticed the API starting to send it.
• docs/models/researchresponse.md never documented warnings — the field
  exists on ResearchResponse and has always parsed correctly; only the docs
  page was missing its row.

Changed

• scripts/check_drift.py recurses nested response schemas — the response
  check previously compared top-level fields only, so drift inside a nested
  object went undetected. Recursion surfaced the two gaps above, which are now
  closed by adding the fields rather than suppressed, so KNOWN_RESPONSE_GAPS
  is empty. It and its mirror KNOWN_SHARED_MODEL_EXTRAS both report an entry
  as stale once the side they excuse catches up, instead of quietly keeping it.
• scripts/check_drift.py recursion hardened — visited was a global
  "already compared this model" cache, so a model reused at two response paths
  backed by different schemas was compared only at the first and drift on later
  branches went unreported. It is now a recursion stack, discarded on exit, so
  cycles still terminate without suppressing sibling branches. The staleness
  check also intersected only with the SDK's fields, which reported a field the
  spec dropped as stale — contradicting the comment above it — and now requires
  the spec to still define the field. Both were found in review, and both are
  covered by tests/test_check_drift.py, the first test coverage that script has
  had.
• scripts/audit_wire.py (new) — walks the raw JSON from a live call next to
  the parsed model and reports any key the model discarded. check_drift.py
  compares the published specs against the models, which cannot see a field the
  API returns but no spec declares; that is how AnswerSearchResult came to drop
  description and thumbnail_url unnoticed. Needs YDC_API_KEY, so it is a
  pre-release check rather than a CI gate. Two keys are recorded in
  KNOWN_WIRE_EXTRAS as observed-but-undeclared rather than modeled:
  results.web[].original_thumbnail_url, and finance-research's top-level
  warnings, which the sibling research spec does declare.

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Full changelog: https://github.com/youdotcom-oss/youdotcom-python-sdk/blob/v3.5.0/CHANGELOG.md