fix: correct sindi n_candidate error message and doc

Fixes: #2708

Description

The SINDI::KnnSearch parameter check validates n_candidate against SPARSE_AMPLIFICATION_FACTOR * k (500x), but the error message printed AMPLIFICATION_FACTOR * k (100x). This mismatch misleads users when tuning the parameter.

Additionally, the SINDI documentation stated that n_candidate = 0 defaults to SPARSE_AMPLIFICATION_FACTOR * topk (500x), which does not match the actual implementation. The code uses ef = max(n_candidate, k), so the default candidate heap size is topk.

Changes

• src/algorithm/sindi/sindi.cpp: use SPARSE_AMPLIFICATION_FACTOR in the error message.
• docs/docs/en/src/indexes/sindi.md: correct the description of n_candidate default behavior and allowed range.
• docs/docs/zh/src/indexes/sindi.md: sync the Chinese documentation.
• tests/test_sindi.cpp: add a test verifying that the error message matches the actual limit (500 * k).

Testing

• Built with make release in the vsaglib/vsag:ubuntu Docker development image.
• Ran the new SINDI test: the boundary case n_candidate = 500 * k succeeds, and n_candidate = 500 * k + 1 fails with a message containing "should be less than 500 * k".
• Ran make fmt and make lint; no new issues introduced.

Checklist

• Code follows the project coding style.
• Added/updated unit tests for the bug fix.
• Updated both English and Chinese documentation.
• Verified the build and tests pass.