feat: Provide a low level Async Interface

[mkaufmann]

Why:
JDBC's synchronous interface is a poor fit for reactive frameworks
(RxJava, Vert.x, Project Reactor) and event-driven architectures.
Teams using these paradigms previously had to block threads or build
custom wrappers around our protocol. Note that this layer has less strong
API stability guarantees compared to the JDBC API as we might still need to
evolve it.

What:
- New async protocol layer under protocol/async/ with CompletionStage-
based iteration over query results, chunks, and query info
- Core abstractions: AsyncIterator, AsyncStreamObserver for gRPC
- Refactored sync iterators to extend SyncIteratorAdapter, reducing
duplication while preserving existing JDBC behavior
- Removed legacy BufferingStreamIterator/Observer in favor of new
async-first design

How:
- AsyncQueryResultIterator: manages full query lifecycle asynchronously
(execute → poll status → fetch chunks)
- AsyncExecuteQueryIterator: handles execute query stream with QueryInfo
tracking
- AsyncStreamObserverIterator: wraps gRPC ClientResponseObserver as
AsyncIterator with backpressure support
- SyncIteratorAdapter: bridges async iterators to blocking Iterator for
JDBC compatibility
- Usage examples demonstrate fully async consumption
patterns including error handling with QueryExceptionHandler that show how it should be used (see com.salesforce.datacloud.jdbc.examples.AsyncQueryResultIteratorTest)

[codecov]

Codecov Report

❌ Patch coverage is 82.84884% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.59%. Comparing base (6b2c170) to head (2a0c343).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../jdbc/protocol/async/core/SyncIteratorAdapter.java	61.53%	13 Missing and 2 partials ⚠️
.../jdbc/protocol/async/AsyncQueryResultIterator.java	84.21%	5 Missing and 4 partials ⚠️
...d/jdbc/protocol/async/AsyncChunkRangeIterator.java	66.66%	5 Missing and 3 partials ⚠️
.../jdbc/protocol/async/core/AsyncStreamObserver.java	86.88%	2 Missing and 6 partials ⚠️
...ud/jdbc/protocol/async/AsyncQueryInfoIterator.java	86.53%	2 Missing and 5 partials ⚠️
...jdbc/protocol/async/AsyncExecuteQueryIterator.java	80.64%	2 Missing and 4 partials ⚠️
.../jdbc/protocol/async/AsyncResultRangeIterator.java	86.66%	4 Missing ⚠️
...oud/jdbc/protocol/async/AsyncRowRangeIterator.java	91.30%	0 Missing and 2 partials ⚠️

❌ Your patch status has failed because the patch coverage (82.84%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #150      +/-   ##
============================================
- Coverage     82.74%   82.59%   -0.16%     
- Complexity     1301     1350      +49     
============================================
  Files           107      112       +5     
  Lines          3918     4062     +144     
  Branches        404      413       +9     
============================================
+ Hits           3242     3355     +113     
- Misses          493      515      +22     
- Partials        183      192       +9     

Components	Coverage Δ
JDBC Core	83.72% <82.84%> (-0.25%)	⬇️
JDBC Main	40.69% <ø> (ø)
JDBC HTTP	91.09% <ø> (ø)
JDBC Utilities	66.07% <ø> (ø)
Spark Datasource	∅ <ø> (∅)

Files with missing lines	Coverage Δ
...force/datacloud/jdbc/core/DataCloudConnection.java	58.45% <100.00%> (+0.29%)	⬆️
...atacloud/jdbc/protocol/AsyncQueryAccessHandle.java	100.00% <100.00%> (ø)
...ce/datacloud/jdbc/protocol/ChunkRangeIterator.java	71.42% <100.00%> (+3.68%)	⬆️
...rce/datacloud/jdbc/protocol/QueryInfoIterator.java	80.00% <100.00%> (-10.00%)	⬇️
...e/datacloud/jdbc/protocol/QueryResultIterator.java	100.00% <100.00%> (+17.64%)	⬆️
...e/datacloud/jdbc/protocol/QuerySchemaAccessor.java	90.00% <100.00%> (+0.52%)	⬆️
...orce/datacloud/jdbc/protocol/RowRangeIterator.java	66.66% <100.00%> (-19.55%)	⬇️
...otocol/async/core/AsyncStreamObserverIterator.java	100.00% <100.00%> (ø)
...oud/jdbc/protocol/async/AsyncRowRangeIterator.java	91.30% <91.30%> (ø)
.../jdbc/protocol/async/AsyncResultRangeIterator.java	86.66% <86.66%> (ø)
... and 6 more

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