[improvement](be) Track and cap inverted index writer memory

[airborne12]

What problem does this PR solve?

Issue Number: close #xxx

Related PRs:

• [improvement](clucene) Report fulltext writer RAM usage doris-thirdparty#393 — clucene-side: expose
  SDocumentsWriter buffered postings via getRAMUsed(). Must merge
  before this PR (this PR bumps contrib/clucene to that commit).
• [feature](inverted index) Introduce SPIMI V4 inverted index storage format #63633 — downstream consumer (SPIMI V4 inverted index
  storage format). Sits on top of this PR.

Problem Summary:

Tighten BE memory tracking and capping for analyzed inverted index
writers (fulltext + Chinese tokenization etc.). Today the BE's
IndexColumnWriter::size() returns 0 for inverted index writers, so
segment_writer.cpp's segment memory estimate misses the CLucene
buffered postings entirely — by far the largest source of resident
bytes during a fulltext flush. Under tight memory pressure (cloud BE
with inverted_index_ram_dir_enable=false), this causes the
segment-builder to overshoot the memory budget and OOM the BE.

Three commits, smallest-first:

1. [improvement](be) Track inverted index writer memory estimate —
   Plumb a real size() reporter through the writer hierarchy. No
   behaviour change yet; just the scaffolding the next two commits
   plug values into.

2. [improvement](be) Account fulltext inverted index writer memory —
   Implement size() for the fulltext path: report
   _null_bitmap.getSizeInBytes() + index_writer->ramSizeInBytes()
   (via the clucene getRAMUsed() exposed by
   doris-thirdparty#393) + ram_directory_memory_size(). The segment
   memory estimate now sees fulltext writer memory.

3. [improvement](be) Cap fulltext index memory without ram dir —
   New config inverted_index_ram_buffer_size_when_ram_dir_disabled
   (default 64 MB). When the column has an analyzer AND
   is_fs_directory(_dir) (i.e. inverted_index_ram_dir_enable=false),
   clamp CLucene's setRAMBufferSizeMB() down from the global
   inverted_index_ram_buffer_size = 512 MB to this new cap. Forces
   more frequent segment flushes in exchange for tighter per-column
   RAM peak — the trade-off cloud users want when RAM dir is
   disabled.

Why a precursor PR

These three are independently useful as memory-tracking +
capping for the existing V1/V2/V3 CLucene writer path. They were
originally bundled in #63633 (the V4 SPIMI PR) as
prerequisites; lifting them out makes the V4 PR smaller and lets
the cloud teams pick up the memory tracking improvements without
waiting for the larger SPIMI refactor to land.

Release note

Add inverted index writer memory tracking and a new cap config
inverted_index_ram_buffer_size_when_ram_dir_disabled (default
64 MB) that limits the CLucene buffered postings size for analyzed
columns when RAM directory is disabled. Reduces BE OOM risk under
fulltext-heavy concurrent loads in cloud mode.

Check List (For Author)

• Test:
  • Unit Test
    (InvertedIndexWriterTest.FullTextStringMemoryEstimateIncludesBufferedPostings
    • *RamBufferCap* tests)
  • Manual test
• Behavior changed:
  • No.
  • Yes — adds a new config that caps CLucene
    setRAMBufferSizeMB when RAM dir is disabled. Default 64 MB.
    Tunable at runtime (mutable bool).
• Does this need documentation:
  • Yes — config will appear in admin doc; doc PR will follow.

[hello-stephen]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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