Add assert_string_matches_format assertion

[Chemaclass]

🤔 Background

Related #177

Adds PHPUnit-style format matching assertions, allowing users to validate strings against format patterns with placeholders instead of writing raw regex.

💡 Changes

• Add assert_string_matches_format and assert_string_not_matches_format assertions
• Support placeholders: %d (digits), %i (signed int), %f (float), %s (non-whitespace), %x (hex), %e (scientific), %% (literal %)
• Add internal bashunit::format_to_regex helper that converts format strings to regex with proper escaping
• Add 11 unit tests covering all placeholders, multiple placeholders, failure messages, and the inverse assertion

[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	126	>= 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}