Add --warnings-collapse-threshold to control warning location collapsing

[V1SHAL421]

Closes #14983

Once a warning message reaches 10 locations, pytest collapses the output to filenames only — dropping the test node IDs entirely. This is counterproductive: the more warnings you have, the more you need to know which tests are responsible. The threshold of 10 is hardcoded with no way to change it per-project.

This solution makes the threshold configurable via --warnings-collapse-threshold (CLI) and warnings_collapse_threshold (INI/TOML). Defaults to 10 so existing behaviour is completely preserved. Set to none to never collapse — always show full test node IDs regardless of count. Invalid values (zero, negative, non-numeric) are rejected at argument-parse time with a clear error message.

Checklist

• Include documentation when adding new features.
• Include new tests or update existing tests when appl
• Allow maintainers to push and squash when merging my commits.
• Add text like closes Allow disabling aggregation of warnings by filename #14983 to the PR description.
• If AI agents were used, they are credited in Co-authored-by commit trailers.
• Create a new changelog file (changelog/14983.feature
• Add yourself to AUTHORS in alphabetical order.

AI assistance

This contribution was developed with the assistance of Claude (Anthropic). All code has been reviewed and understood by the author.