fix(connectors): support default credential provider chain for iceberg sink

[gomnitrix]

Which issue does this PR close?

Closes #2911

Rationale

Production environments prefer the default credential provider chain over static keys, which the Iceberg Sink connector currently lacks support for.

What changed?

The Iceberg Sink connector previously failed to initialize if store_access_key_id and store_secret_access_key were missing from the config, blocking the use of standard identity-based auth. This fix makes these config fields optional, allowing the underlying OpenDAL seamlessly fall back to the default credential provider chain. An isolated integration test was also added to verify this fallback behavior using environment variables.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

1. Which tools? Gemini 3.1 Pro
2. Scope of usage? Paired to implement the iceberg_sink_uses_default_credential_chain integration test.
3. How did you verify the generated code works correctly? Verified the test logic and ensured it correctly polls Iceberg snapshots without explicit static access key config.
4. Can you explain every line of the code if asked? yes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.75%. Comparing base (411a697) to head (b324786).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #3045   +/-   ##
=========================================
  Coverage     71.74%   71.75%           
  Complexity      943      943           
=========================================
  Files          1121     1121           
  Lines         93800    93826   +26     
  Branches      71124    71150   +26     
=========================================
+ Hits          67301    67323   +22     
+ Misses        23863    23862    -1     
- Partials       2636     2641    +5     

Components	Coverage Δ
Rust Core	72.36% <100.00%> (+<0.01%)	⬆️
Java SDK	62.30% <ø> (ø)
C# SDK	67.64% <ø> (ø)
Python SDK	81.43% <ø> (ø)
Node SDK	91.31% <ø> (ø)
Go SDK	38.68% <ø> (ø)

Files with missing lines	Coverage Δ
core/connectors/sinks/iceberg_sink/src/lib.rs	100.00% <ø> (ø)
core/connectors/sinks/iceberg_sink/src/props.rs	95.34% <100.00%> (+7.11%)	⬆️
core/connectors/sinks/iceberg_sink/src/sink.rs	100.00% <ø> (ø)

... and 15 files with indirect coverage changes

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