fix(client): cap batch fetch concurrency

[chaliy]

Motivation

• Prevent unbounded caller-controlled concurrency in the public batch APIs that could exhaust runtime resources and enable application-level DoS.

Description

• Add MAX_BATCH_CONCURRENCY constant set to 20 to impose a conservative upper bound.
• Clamp effective concurrency with .clamp(1, MAX_BATCH_CONCURRENCY) in batch_fetch_with_options instead of only enforcing a minimum.
• Add a unit test test_batch_fetch_caps_caller_concurrency that passes usize::MAX to verify oversized caller input is safely handled.

Testing

• Ran targeted unit tests for batch fetching; the following tests passed: test_batch_fetch_caps_caller_concurrency, test_batch_fetch_respects_concurrency_limit, test_batch_fetch_multiple_urls, test_batch_fetch_partial_failure, and test_batch_fetch_empty_input (all OK).
• Executed cargo test --workspace batch_fetch which completed successfully for the relevant test suites.

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Codex Task