Optimize ExecutionAttributes

[alextwoods]

Optimize ExecutionAttributes to reduce resizes and reduce hash computation cost

Motivation and Context

Fixes: #6846

Modifications

There are two modifications:

1. Increase the initial capacity to 64. In most requests I tested we had around 45 execution attributes, so this prevents resizing for those requests.
2. Switch to using IdentityHashMap, which uses == instead of the string hash code.

IdentityHashMap uses == instead of .equals() for key comparison. This is safe for ExecutionAttributes because all keys are enforced singletons. ExecutionAttribute.ensureUnique() throws IllegalArgumentException if you try to create a second instance with the same name. Every ExecutionAttribute in the SDK is declared static final.

The change is internal. The field type is still Map<ExecutionAttribute<?>, Object>, and getAttributes() wraps it in Collections.unmodifiableMap(), so callers are unaffected.

Testing

All existing tests.

Performance

The combined optimization (IdentityHashMap + capacity 64)
provides a modest but measurable improvement:

• Eliminates 1 HashMap resize per request (saves ~0.5-1% CPU)
• Eliminates String-based hashCode computation for all attribute lookups (saves ~0.3% CPU)
• Reduces Node object allocation (IdentityHashMap uses flat array)
• Total: ~1-2% CPU reduction in the ExecutionAttributes hot path

ops/cpu-second for a high throughput dynamodb workload (running on a c7g.8xlarge):

optimized: 3255.4414101
baseline: 3186.2756065
Improvement: 2.17%

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.

License

• I confirm that this pull request can be released under the Apache 2 license

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