fix(knowledge): bound chunking separators so one config can't stall processing - #6735

…rocessing

`chunkingStrategyOptionsSchema.separators` accepted an arbitrary-length array
of arbitrary-length strings, next to a `pattern` field already capped at 500
chars. `RecursiveChunker` splits the whole document once per separator and
walks the list from the top for every oversized fragment, so a persisted
config with thousands of non-matching separators cost seconds of synchronous
CPU on every later document upload — work neither the processing `Promise.race`
timeout nor the after-the-fact chunk-count cap can interrupt. Measured on a
21.3 MB document: 632 ms at 100 separators, 6.1 s at 1000, 36.8 s at 5000.

- Bound `separators` to 32 entries of at most 100 characters on the write path.
  The largest built-in recipe (markdown) uses 16, so hand-tuned lists still fit.
- Keep the stored/read shape tolerant, so a config written before the bound
  still lists instead of failing response validation.
- Clamp in `RecursiveChunker` too, with a warning, so an already-persisted
  oversized list cannot reach the split loop. An over-long separator is dropped
  rather than truncated: a truncated separator matches where the configured one
  never did, silently re-cutting the document, while dropping it behaves like a
  separator that finds no match. A list left empty falls back to the recipe.
- Walk non-matching separators iteratively instead of recursing, so stack depth
  no longer tracks the separator count. Verified behavior-preserving against the
  previous implementation over 4000 randomized configs — byte-identical output.
- Validate in the create-base modal so the limit surfaces inline.

After the fix the same 21.3 MB document costs ~300 ms at every separator count.

- The separator refines ran for every strategy, but the field only renders for
  `recursive` and only that strategy submits it, so a value left behind by a
  strategy switch could block submit with no visible field to clear. Gated the
  same way the regex-pattern refine already is.
- Use absolute imports in the chunker test, per the repo convention.