fix(parser): replace expect_expression panic with try_into_expression…

[tkshsbcue]

fix(parser): replace expect_expression panic with try_into_expression error handling

Summary

Replaces all uses of expect_expression() with try_into_expression() in the parser, and removes the panicking expect_expression() method. When the parser encounters arrow-function parameter list syntax in a context that requires an expression (e.g. conditional operator a ? b : c, call expression foo(), optional chaining a?.b), it now returns a recoverable parse error instead of panicking.

Motivation

The FormalParameterListOrExpression type represents the grammar ambiguity between (a, b) as a comma expression vs (a, b) as arrow-function parameters. In contexts like the conditional operator (x ? y : z), call expressions (f()), and optional chaining (a?.b), the left-hand side must be an expression—arrow params are invalid. Previously, call sites used expect_expression(), which panics with "Unexpected arrow-function arguments" when the parser produced a FormalParameterList instead of an Expression. This can occur with edge-case input such as (a) ? 1 : 2 when (a) is parsed as a single arrow param. For Boa as an embedded engine or in tooling, panics are unacceptable; the host expects a parse error. The try_into_expression() method already existed and returns Err(Error::General { ... }) with a helpful message ("invalid arrow-function arguments (parentheses around the arrow-function may help)"). Replacing expect_expression() with try_into_expression()? propagates that error instead of panicking, aligning with Boa's stability goals.

Changes

Category	Description
Replaced	lhs.expect_expression() with lhs.try_into_expression()? in conditional expression parser
Replaced	member.expect_expression() with member.try_into_expression()? in left-hand side (call expression)
Replaced	lhs.expect_expression() with lhs.try_into_expression()? in left-hand side (optional expression)
Removed	expect_expression() method from FormalParameterListOrExpression to prevent future misuse

Technical Details

• Files modified: core/parser/src/parser/expression/assignment/conditional.rs, core/parser/src/parser/expression/left_hand_side/mod.rs, core/parser/src/parser/expression/fpl_or_exp.rs
• Lines changed: ~15 lines across 3 files
• Behavioral impact: Invalid input that previously caused a panic now produces Err(Error::General { message: "invalid arrow-function arguments (parentheses around the arrow-function may help)", position }). Valid input is unchanged.

Testing

• cargo test -p boa_parser — all 296 tests pass
• cargo test -p boa_engine --lib — all 921 tests pass
• cargo clippy — no warnings

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,687	49,687	0
Ignored	2,262	2,262	0
Failed	1,014	1,014	0
Panics	0	0	0
Conformance	93.81%	93.81%	0.00%

Tested main commit: 28648a3207b482955f3f93935de1b181ab72c477
Tested PR commit: 0eedab4650bc2076d31857eb933a37162e6c3431
Compare commits: 28648a3...0eedab4

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.97%. Comparing base (6ddc2b4) to head (0eedab4).
⚠️ Report is 776 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4923       +/-   ##
===========================================
+ Coverage   47.24%   57.97%   +10.73%     
===========================================
  Files         476      557       +81     
  Lines       46892    60931    +14039     
===========================================
+ Hits        22154    35325    +13171     
- Misses      24738    25606      +868     

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[tkshsbcue]

@nekevss i hope you can review my PR
Thanks!