refactor: reduce lodash footprint (follow-up to #1509)

[s-lukashenka-micoworks]

Summary

Follow-up to the discussion in #1509. That issue proposed swapping lodash for es-toolkit; @sserrata closed it in favor of three lower-risk steps and noted PRs for items 1–3 would be welcome. This PR implements all three.

Each item is a separate commit so they can be reviewed (or dropped) independently:

1. build(deps): bump lodash to ^4.18.1 — picks up the security patches released in the lodash 4.18.x line.
2. refactor: replace lodash find/flatten/uniq/isEmpty with native JS — removes the four utilities that have direct native equivalents:
   • uniq → [...new Set(...)] (openapi.ts, sidebars/index.ts)
   • flatten → Array.prototype.flat() (DiscriminatorTabs, SchemaTabs)
   • find → Array.prototype.find() (languages.ts)
   • isEmpty → Object.keys(x).length === 0 (createSchema.ts, Schema)
3. refactor: use lodash subpath import for kebabCase in sidebars — switches the one remaining barrel import (import { kebabCase } from "lodash") to the subpath form so the rest of lodash can be tree-shaken. Every other lodash usage already imports from a subpath.

The remaining lodash utilities (merge, mergeWith, cloneDeep, kebabCase, unionBy) are kept and could be vendored later (item 4) if the project wants to drop the dependency entirely.

Notes on correctness

• The two isEmpty call sites are both guarded by a preceding truthiness check (if (!additionalProperties) return ...), so the value is always a non-null object at the call — Object.keys(x).length === 0 is equivalent there.
• The find replacement keeps optional chaining (arr1?.find(...)) to preserve lodash's tolerance of a nullish collection.
• lodash.flatten flattens a single level, which matches Array.prototype.flat()'s default depth of 1.

Testing

• yarn build (tsc) passes for both docusaurus-plugin-openapi-docs and docusaurus-theme-openapi-docs.
• All test suites covering the changed files pass (sidebars, languages, openapi, webhooks).
• One unrelated suite (createSchema.test.ts) fails, but it fails identically on a clean main checkout (an allof-merge issue) — it is not affected by these changes.

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