api: Summarize EquivalentAddressGroup toString

[stagegrowth]

Background

EquivalentAddressGroup.toString() currently prints the full address list, which can become extremely large and noisy for big address sets. This makes logs and test output hard to read and can obscure useful information.

Changes

This PR adds a hard cap to the number of addresses rendered in toString() and summarizes the remainder (e.g., “... N more”). It also adds focused unit tests to validate the large-list and boundary behaviors, and refactors related CallOptionsTest assertions using a small Truth subject for clearer equivalence checks.

Purpose

Keep toString() output readable even with very large address lists
Ensure the summarization behavior is covered by unit tests
Improve clarity of CallOptions equivalence assertions in tests

Note

The summary threshold is defined as a private constant and is only used for output formatting. It does not change the underlying address data or behavior—only the string representation.

Fixes #12593

[linux-foundation-easycla]

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