fix: eliminate global logger side effects in mapper package

[tekenstam]

Summary

• Change MapperArguments.Validate() to return error instead of calling log.Fatal(), enabling library consumers to handle validation failures gracefully (fixes Bug: Validate() calls log.Fatal() preventing library error handling #143)
• Replace the global log.SetOutput(io.Discard) call in New() with a per-instance *log.Logger field on AuthMapper, so creating a silent mapper no longer silences logging for the entire process (fixes Bug: New() with isCommandline=false disables logging globally #144)
• Convert WithRetry from a package-level function to an AuthMapper method so it uses the instance logger
• Remove the unused LoggingEnabled field and the init() function that mutated global log flags
• Update all callers (Upsert, Remove, RemoveByUsername, Get) to check and propagate validation errors and use the instance logger
• Add unit tests for both fixes

Test plan

• All existing unit tests pass (go test ./...)
• New tests cover every validation branch: invalid retry count, missing role/user ARN, mutually exclusive flags, missing username, invalid format, missing map selection, and successful validation
• New tests verify per-instance logger isolation (two AuthMapper instances with different logging settings don't interfere)
• Verify CI passes on this PR

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.94737% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.98%. Comparing base (5cf8555) to head (969f0c2).

Files with missing lines	Patch %	Lines
pkg/mapper/remove.go	63.63%	2 Missing and 2 partials ⚠️
pkg/mapper/get.go	33.33%	1 Missing and 1 partial ⚠️
pkg/mapper/upsert.go	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
+ Coverage   68.97%   69.98%   +1.01%     
==========================================
  Files          12       12              
  Lines         593      593              
==========================================
+ Hits          409      415       +6     
+ Misses        165      162       -3     
+ Partials       19       16       -3     

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