fix(async): pooledMap surfaces errors thrown by the input iterable

[LeSingh1]

Fixes #6716.

When an async iterable passed to pooledMap throws (rather than a transformation), the catch block discarded the error and built an AggregateError populated solely from the executing transformations. With no transformation rejections in flight, callers saw only:

AggregateError: Cannot complete the mapping as an error was thrown from an item
    errors: []

The underlying cause was unrecoverable.

Repro from the issue:

import { pooledMap } from "jsr:@std/async/pool";

async function* errorThrowingIterator() {
  throw new Error("Iterator failed on first step!");
  yield 1;
}

const results = pooledMap(2, errorThrowingIterator(), async (i) => i * 2);
for await (const _ of results) {}
// AggregateError, errors: [] — the real cause is lost

After the patch the iterable's error rides through in AggregateError.errors:

message: Cannot complete the mapping as an error was thrown from an item
errors:  [ "Iterator failed on first step!" ]

Implementation: bind the catch's value as iterError and errors.push(iterError) only when it isn't already in errors. The Promise.race rejections from the executing pool are already surfaced via the existing allSettled walk, so the includes check prevents the common transformation-rejection path from duplicating its own reason.

Tests:

• New pooledMap() surfaces errors thrown by the input iterable (#6716) pins the iterable-throws case.
• The existing pooledMap() handles errors test (which exercises Promise.race rejections, not iterable throws) still produces exactly two rejections, confirming no double-counting.

All 6 pool_test.ts tests pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (cdf74a8) to head (d5ee03f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7169   +/-   ##
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  Coverage   94.57%   94.57%           
=======================================
  Files         636      636           
  Lines       52142    52143    +1     
  Branches     9401     9402    +1     
=======================================
+ Hits        49315    49316    +1     
  Misses       2249     2249           
  Partials      578      578           

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