docs: document scrape query limits in request API

[zeitlinger]

Summary

• document the default implementation values maxQueryStringLength = 64 * 1024 and maxQueryParameterCount = 1024 in PrometheusHttpRequest JavaDoc
• clarify that these values are fixed implementation limits, not runtime configuration
• clarify that invalid or excessive query parameters produce HTTP 400

Follow-up to prometheus/client_java#2334 and discussion_r3752169308.

Validation

• mise run lint:fix (blocked by the pre-existing 403 response for the README CNCF Slack link; the push hook passed using a temporary local-only README substitution, restored before completion)
• mise run build
• ./mvnw -pl prometheus-metrics-exporter-common test -Dcoverage.skip=true