feat: add rule E3697 to validate Lambda env var 4KB size limit

[raajheshkannaa]

Closes #1945

Summary

• New rule E3697 that validates AWS::Lambda::Function environment variables do not exceed the 4KB (4096 bytes) limit
• Sums byte lengths of all keys and string values in Environment.Variables
• Reports the actual total size in the error message for easy debugging
• Follows existing CfnLintKeyword pattern used by other Lambda rules

Changes

• src/cfnlint/rules/resources/lmbd/FunctionEnvironmentSize.py: New rule
• test/unit/rules/resources/lmbd/test_function_environment_size.py: 5 parametrized tests

Test plan

• All 98 Lambda rule tests pass (93 existing + 5 new)
• Edge cases: empty vars (pass), exactly 4096 bytes (pass), 4097 bytes (fail)
• Many small keys exceeding limit detected correctly

[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.44%. Comparing base (0bbabc9) to head (1f96080).
⚠️ Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
...nt/rules/resources/lmbd/FunctionEnvironmentSize.py	92.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4405      +/-   ##
==========================================
- Coverage   93.54%   93.44%   -0.10%     
==========================================
  Files         418      423       +5     
  Lines       14145    14350     +205     
  Branches     2817     2865      +48     
==========================================
+ Hits        13232    13410     +178     
- Misses        567      582      +15     
- Partials      346      358      +12     

Flag	Coverage Δ
unittests	93.44% <92.00%> (-0.09%)	⬇️

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