fix(flight-sql): resolve end-of-stream mid-frame error in Flight SQL integration test

[CritasWang]

修复 Flight SQL 集成测试中的 "end-of-stream mid-frame" HTTP/2 帧截断错误。

Root cause / 根本原因:
The gRPC default thread pool executor fails to properly handle subsequent RPCs on the same HTTP/2 connection in the DataNode JVM environment, where standalone Netty JARs coexist with grpc-netty bundled in the fat jar.

DataNode JVM 环境中，gRPC 默认线程池执行器无法正确处理同一 HTTP/2 连接上
的后续 RPC 调用。根因是类路径上独立的 Netty JAR 与 fat jar 中捆绑的
grpc-netty 产生冲突。

Fix / 修复方案:

1. directExecutor() — run gRPC handlers in the Netty event loop thread, bypassing the default executor's thread scheduling issues (关键修复)
2. flowControlWindow(1MB) — explicit HTTP/2 flow control prevents framing errors when duplicate Netty JARs coexist on the classpath
3. Exclude io.netty from fat jar POM — use standalone Netty JARs already on the DataNode classpath instead of bundling duplicates

Additional bug fixes / 其他修复:

• TsBlockToArrowConverter: fix NPE when getColumnNameIndexMap() returns null for SHOW DATABASES queries (回退到列索引)
• FlightSqlAuthHandler: add null guards in authenticate() and appendToOutgoingHeaders() for CallHeaders with null internal maps
• FlightSqlAuthHandler: rewrite as CallHeaderAuthenticator with Bearer token reuse and Basic auth fallback
• FlightSqlSessionManager: add user token cache for session reuse
• IoTDBFlightSqlProducer: handle non-query statements (USE, CREATE, etc.) by returning empty FlightInfo, use TicketStatementQuery protobuf format

Test changes / 测试改动:

• Use fully qualified table names (database.table) instead of USE statement to keep each test to one GetFlightInfo + one DoGet RPC per connection
• All 5 integration tests pass: testShowDatabases, testQueryWithAllDataTypes, testQueryWithFilter, testQueryWithAggregation, testEmptyResult