fix: restore native Date fallback when strictParsing is false

[martijnrusschen]

Summary

• Restore the native Date parsing fallback that was present in v7.x but accidentally removed in v8.0.0
• When strictParsing is false (the default), flexible input formats like 12/05/2025 or 2025-12-16 3:31:01 PM are now parsed even when the dateFormat prop specifies a different format

Problem

In v8.0.0, the parseDate function was refactored and the native new Date(value) fallback was removed. This caused a regression where users could no longer type dates in flexible formats - only exact matches to the dateFormat prop were accepted.

Before v8 (working):

<DatePicker dateFormat="yyyy-MM-dd hh:mm aa" />
// User could type: "12/05/2025" ✅
// User could type: "2025-12-16 3:31:01 PM" ✅

v8.0.0+ (broken):

<DatePicker dateFormat="yyyy-MM-dd hh:mm aa" />
// User must type exact format: "2025-12-16 03:31 PM" 
// Other formats rejected ❌

Solution

Add back the native Date fallback when strictParsing is false. The fallback only triggers when:

1. strictParsing is false (default)
2. The input is at least 8 characters (to avoid parsing partial inputs like "03/" or "2000")
3. date-fns parsing with the specified format(s) failed

Test plan

• Added tests for native Date fallback behavior
• All 1471 existing tests pass
• Verified partial date inputs still work for calendar navigation

Fixes #6164

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.31%. Comparing base (1e9e151) to head (972a4b5).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6172      +/-   ##
==========================================
+ Coverage   99.26%   99.31%   +0.05%     
==========================================
  Files          30       30              
  Lines        3801     3805       +4     
  Branches     1652     1654       +2     
==========================================
+ Hits         3773     3779       +6     
+ Misses         27       25       -2     
  Partials        1        1              

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