fix(es/minifier): preserve undefined parameter defaults

[labor0-bot]

Description:

Preserve root function-parameter defaults that evaluate to undefined. Removing
them changes Function.length and turns non-arrow functions with an otherwise
simple parameter list into functions with mapped arguments.

The pure minifier now visits parameter-pattern children without simplifying the
root pattern. This retains observable parameter defaults while preserving the
existing simplification for nested and non-parameter patterns.

Adds SWC-owned issue fixtures covering function arity, mapped arguments, and a
non-parameter destructuring control.

Related issue (if exists):

Closes #12203

Verification:

• UPDATE=1 cargo test -p swc_ecma_minifier
• cargo test -p swc_ecma_minifier
• ./scripts/test.sh
• ./scripts/exec.sh
• cargo fmt --all
• cargo clippy --all --all-targets -- -D warnings is blocked because hosted-runner policy disables git submodule update --init --recursive; required HTML5lib and Test262 fixture submodules are unavailable.

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⚠️ No Changeset found

Latest commit: f009735

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Labor0 task status

• Status: Waiting for review or merge
• Current work: None
• Last updated: 2026-09-05 12:59 UTC

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[codspeed]

Merging this PR will not alter performance

✅ 200 untouched benchmarks
⏩ 61 skipped benchmarks¹

---

_{Comparing fix-12203-preserve-undefined-parameter-defaults (f009735) with main (dcbcf93)²}

Footnotes

1. 61 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (4a73df1) during the generation of this report, so dcbcf93 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩