fix: raise docCache per-entry cap to 5MB with size warnings

llms-full.txt files (GL JS 1.6MB, iOS Nav 696KB, Style Spec 466KB)
previously fell under or near the 2MB hard cap. Raising to 5MB lets
all current full-content files cache cleanly.

Add warnings so operators can see large entries in logs:
- entries 1MB–5MB: warn and cache
- entries >5MB: warn and reject (instead of silently dropping)

Update BatchGetDocumentsTool test to use 6MB as the "too large" value.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>