[w3c/IndexedDB] Restore DataError for equal keys with an open bound (PR #514)

# Restore DataError for equal keys with an open bound

Closes #510

## Summary

Add the missing `IDBKeyRange.bound()` step: if the converted keys are equal and either open flag is true, throw a `DataError` DOMException before creating a range. Equal keys with both bounds closed remain valid.

This restores the condition in [IndexedDB 1.0](https://www.w3.org/TR/2015/REC-IndexedDB-20150108/#widl-IDBKeyRange-bound-IDBKeyRange-any-lower-any-upper-boolean-lowerOpen-boolean-upperOpen). Thanks to @bcardarella for reporting the omission.

## Tests

* [x] Confirmed there are no Bikeshed errors or warnings: Bikeshed 7.2.1, before and after this change.
* [ ] Modified Web platform tests: WPT_PR_URL — replace after opening the companion PR.

The companion patch adds three equal-key/open-bound DataError cases and a closed/closed success control to `IndexedDB/idbkeyrange_incorrect.any.js`.

Locally, the changed file and the neighboring `idbkeyrange.any.js` and `idbkeyrange-includes.any.js` pass in headless Chrome 156.0.8078.47 and Firefox 159.0.1: 12 generated tests, 128 passing subtests per browser, across window, dedicated worker, shared worker and service worker. WPT lint passes for the changed file.

## Implementation status

These are observations, not implementation commitments:

* Chromium: the new cases pass locally in Chrome 156.0.8078.47.
* Gecko: the new cases pass locally in Firefox 159.0.1.
* WebKit: not run locally; #510 reports matching behavior.

No implementation change or commitment is requested by this PR.


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  https://github.com/w3c/IndexedDB/pull/514

-- Commit Summary --

  * Restore DataError for equal keys with an open bound

-- File Changes --

    M index.bs (3)

-- Patch Links --

https://github.com/w3c/IndexedDB/pull/514.patch
https://github.com/w3c/IndexedDB/pull/514.diff

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