fix(jsregexp): convert panics to EngineError::Panic using js_expect

[KaustubhOG]

Part of #3241

Converts all 17 panics in jsregexp.rs to EngineError::Panic using
js_expect introduced in #4828.

No public API signatures were changed — only 1 file modified.

Patterns converted:

• .as_object().expect() → .as_object().js_expect()?
• .map(|v| v.as_boolean().expect()) → .and_then(|v| v.as_boolean().js_expect().map_err(Into::into))
• .map(|v| v.as_string().expect()...to_std_string().expect()) → .and_then(|v| v.as_string().js_expect()?.to_std_string().map_err(...))
• nested .expect() calls in exec() converted using ? inside .and_then()

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,862	52,862	0
Passed	49,627	49,627	0
Ignored	2,263	2,263	0
Failed	972	972	0
Panics	0	0	0
Conformance	93.88%	93.88%	0.00%

Tested PR commit: a4ea5e7c1633d82ae92a9ab336c67b14e01da397

[codecov]

Codecov Report

❌ Patch coverage is 12.76596% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.13%. Comparing base (6ddc2b4) to head (a4ea5e7).
⚠️ Report is 756 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/object/builtins/jsregexp.rs	12.76%	41 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4846      +/-   ##
==========================================
+ Coverage   47.24%   57.13%   +9.89%     
==========================================
  Files         476      554      +78     
  Lines       46892    60484   +13592     
==========================================
+ Hits        22154    34558   +12404     
- Misses      24738    25926    +1188     

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