Fix shell crash when piping assignments into commands (#22336)

## Summary
- Fixes crash when running shell commands with variable assignments
piped to other commands
- Resolves #15714

## Problem
The shell was crashing with "Invalid tag" error when running commands
like:
```bash
bun exec "FOO=bar BAR=baz | echo hi"
```

## Root Cause
In `Pipeline.zig`, the `cmds` array was allocated with the wrong size:
- It used `node.items.len` (which includes assignments)
- But only filled entries for actual commands (assignments are skipped
in pipelines)
- This left uninitialized memory that caused crashes when accessed

## Solution
Changed the allocation to use the correct `cmd_count` instead of
`node.items.len`:
```zig
// Before
this.cmds = if (cmd_count >= 1) bun.handleOom(this.base.allocator().alloc(CmdOrResult, this.node.items.len)) else null;

// After  
this.cmds = if (cmd_count >= 1) bun.handleOom(this.base.allocator().alloc(CmdOrResult, cmd_count)) else null;
```

## Test plan
✅ Added comprehensive regression test in
`test/regression/issue/15714.test.ts` that:
- Tests the exact case from the issue
- Tests multiple assignments
- Tests single assignment
- Tests assignments in middle of pipeline
- Verified test fails on main branch (exit code 133 = SIGTRAP)
- Verified test passes with fix

🤖 Generated with [Claude Code](https://claude.ai/code)

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Zack Radisic <56137411+zackradisic@users.noreply.github.com>

src/shell/states/Pipeline.zig

@@ -95,7 +95,7 @@ fn setupCommands(this: *Pipeline) ?Yield {
         break :brk i;
     };
 
-    this.cmds = if (cmd_count >= 1) bun.handleOom(this.base.allocator().alloc(CmdOrResult, this.node.items.len)) else null;
+    this.cmds = if (cmd_count >= 1) bun.handleOom(this.base.allocator().alloc(CmdOrResult, cmd_count)) else null;
     if (this.cmds == null) return null;
     var pipes = bun.handleOom(this.base.allocator().alloc(Pipe, if (cmd_count > 1) cmd_count - 1 else 1));