feat(doubles): add assert_have_been_called_nth_with assertion

[Chemaclass]

🤔 Background

Related #172

Adds a dedicated assertion for verifying arguments on the Nth invocation of a spy, providing a clearer API than the hidden optional index parameter in assert_have_been_called_with.

💡 Changes

• Add assert_have_been_called_nth_with with explicit API: assert_have_been_called_nth_with <n> <command> <expected_args>
• Include index bounds checking with clear error messages when Nth call doesn't exist
• Add 3 unit tests covering success, failure, and invalid index cases

[github-actions]

✅ Contributor Report

User: @Chemaclass
Status: Passed (13/13 metrics passed)

Metric	Description	Value	Threshold	Status
PR Merge Rate	PRs merged vs closed	93%	>= 30%	✅
Repo Quality	Repos with ≥100 stars	6	>= 0	✅
Positive Reactions	Positive reactions received	157	>= 1	✅
Negative Reactions	Negative reactions received	0	<= 5	✅
Account Age	GitHub account age	4569 days	>= 30 days	✅
Activity Consistency	Regular activity over time	108%	>= 0%	✅
Issue Engagement	Issues with community engagement	0	>= 0	✅
Code Reviews	Code reviews given to others	173	>= 0	✅
Merger Diversity	Unique maintainers who merged PRs	8	>= 2	✅
Repo History Merge Rate	Merge rate in this repo	89%	>= 0%	✅
Repo History Min PRs	Previous PRs in this repo	125	>= 0	✅
Profile Completeness	Profile richness (bio, followers)	100	>= 0	✅
Suspicious Patterns	Spam-like activity detection	0	N/A	✅

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_{Contributor Report evaluates based on public GitHub activity. Analysis period: 2025-02-21 to 2026-02-21}