feat(qwp): add deferred commit for transactional ingestion

[bluestreak01]

Fixes #7143

Tandem with questdb/java-questdb-client#32.

Summary

Adds FLAG_DEFER_COMMIT (0x01) to the QWP ingress protocol. When a
client sets this flag on a message, the server appends rows to WAL
writers but skips the commit. The commit fires on the next message
without the flag. This allows clients to split a logical batch across
multiple messages that individually fit within the server's recv buffer,
removing the hard coupling between http.recv.buffer.size and maximum
transaction size.

Server changes

• QwpConstants: add FLAG_DEFER_COMMIT = 0x01.
• QwpIngressProcessorState: add isDeferCommit() to read the flag
  from the message header, and clearMessageState() to reset
  per-message parsing state without rolling back WAL rows.
• QwpIngressUpgradeProcessor.handleBinaryMessage(): skip commit()
  when FLAG_DEFER_COMMIT is set; call clearMessageState() instead
  of clear() in the finally block so accumulated WAL rows survive.
  On error, the full clear() still fires to roll back all deferred
  rows.

Client changes (java-questdb-client submodule)

• QwpConstants: add FLAG_DEFER_COMMIT = 0x01.
• QwpWebSocketEncoder: add setDeferCommit() to set/clear the flag.
• QwpBufferWriter / all implementations: add patchByte().
• QwpWebSocketSender.flushPendingRows(boolean deferCommit): when
  deferCommit is true, the encoded message carries FLAG_DEFER_COMMIT.
  When the batch exceeds serverMaxBatchSize, flushPendingRowsSplit()
  sends each table as a separate deferred message with the last one
  committing (or deferring, if the caller requested defer).
• QwpWebSocketSender.sendCommitMessage(): sends an empty QWP message
  (0 table blocks, no defer flag) to trigger the server-side commit
  when all pending rows were already sent via deferred auto-flushes.
• Sender builder: add transaction=on|off connect-string key and
  transactional(boolean) builder method. When enabled, auto-flush
  sends with FLAG_DEFER_COMMIT and only explicit flush() triggers
  the commit.

Protocol spec update

• Updated the flags byte table (bit 0 is now FLAG_DEFER_COMMIT).
• Added "Deferred commit" section covering wire semantics, error
  handling, backward compatibility, and implementation recommendations.
• Updated OK response docs to mention tableCount=0 for deferred
  messages.

Benchmark

• Added QwpEquitiesL1Benchmark: 10M rows of equities L1 market data
  (500 symbols, 15 exchanges, bid/ask/last prices, volumes) ingested
  with transaction=on — all rows stream via deferred auto-flushes and
  commit once at the end.

Backward compatibility

Old servers ignore the unknown flag bit (it occupies a previously
reserved position). Each message commits independently — more commits
than intended, but no data loss. The feature degrades gracefully.

Error handling

• Parse error or schema mismatch on a deferred message: the server
  calls clear(), rolling back all accumulated WAL rows from the
  entire deferred group. The client must re-send the full sequence.
• Client disconnect mid-sequence: close() frees tudCache, rolling
  back uncommitted WAL rows. The client's store-and-forward replays
  all unacknowledged messages on reconnect.

Test plan

• cd java-questdb-client && mvn -pl core test -- full client suite
  passes
• mvn -pl core compile -DskipTests -- server compiles
• Full server test suite (CI)
• E2E: run QwpEquitiesL1Benchmark against a local server, verify
  10M rows committed atomically on flush()

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[PR Coverage check]

😍 pass : 49 / 64 (76.56%)

file detail

	path	covered line	new line	coverage
🔵	io/questdb/cutlass/qwp/server/QwpUdpReceiver.java	0	1	00.00%
🔵	io/questdb/cutlass/qwp/server/QwpTudCache.java	13	25	52.00%
🔵	io/questdb/PropServerConfiguration.java	3	5	60.00%
🔵	io/questdb/cutlass/qwp/server/QwpIngressProcessorState.java	18	18	100.00%
🔵	io/questdb/cutlass/qwp/server/QwpIngressUpgradeProcessor.java	13	13	100.00%
🔵	io/questdb/PropertyKey.java	1	1	100.00%
🔵	io/questdb/cutlass/http/DefaultHttpServerConfiguration.java	1	1	100.00%