Fix flaky view_to_iframe and django_form submit tests

[Archmonger]

Description

Umbrella PR that fixes the flaky tests surfacing in Test / python-source CI. Two independent flakes are addressed below, plus a probe of the whole suite under load to confirm nothing else is flaky.

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1. test_component_view_to_iframe_* (product change: eager iframe)

Root cause. view_to_iframe rendered the <iframe> with loading="lazy", so the browser only fetched the iframe's document once it scrolled close to the viewport. These iframes sit below the fold of the home page and the shared Playwright page is already sitting on /, so navigate_to_page performs no navigation and the container is never scrolled. Under CI's unpinned Chromium the deferred fetch could exceed the 30s wait_for, so the iframe's [data-success=...] element never appeared.

Fix. Drop the forced loading="lazy" so iframes load eagerly (the HTML default); the iframe document is then fetched as part of the parent page's initial load. Lazy behavior remains opt-in via extra_props={"loading": "lazy"} (merged after the defaults). Verified: all 6 iframe documents load immediately after goto(wait_until="load") at 1280×720 with no scrolling.

2. test_form_* (test change: retry empty/form submits)

Root cause. DjangoForm registers a native submit listener that calls preventDefault() and forwards form data over the WebSocket, but the listener is attached in componentDidMount and can momentarily lag the rendered <form> under load. A click landing during that window performs a native form submission: the browser reloads the page, which resets all ReactPy state so the awaited callback never fires. This matches the CI signature — a second Starting client... with no preceding Disconnected!/Reconnecting... (a WebSocket reconnect logs those; a page reload does not). The pre-existing retry guard only covered the later filled re-submit of the event forms, leaving every empty-submit assertion exposed.

Fix. Generalize the existing retry guard into a shared _retry_submit_until(selector, fill=..., submit_selector=...) helper and route every bare "submit, then wait for the error state" assertion through it:

• test_form_sync_events — empty-submit #error (the reported failure) and the filled re-submit
• test_form_async_events — same two paths
• test_form_basic — empty-submit .errorlist
• test_form_bootstrap — empty-submit .invalid-feedback (uses button[type=submit])
• test_form_orm_model — empty-submit .errorlist

Re-clicking the submit until the awaited state surfaces makes each interaction deterministic instead of racing the listener mount. Test-only; no product code touched.

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Testing

• ruff check / ruff format --check and pyright src pass (0 errors).
• All 5 form tests: green solo and across repeated CPU-contention load runs.
• Full test_components.py under CPU-contention load: 74 passed (exit 0), several times over.
• CI on the iframe-fix commit (f41bda5) was fully green (Test + CodeQL).