Validate UUIDs in LiveView mounts and search filters (Sentry LIGHTNING-3B)

User story

Issue #4588 covers UUID validation across the REST API controllers (PR #4591 is an external contributor's attempt at it). This issue covers the remaining places Ecto.Query.CastError still surfaces from Sentry LIGHTNING-3B that #4588 explicitly scopes out: LiveView route params and search/filter inputs.

Details

LIGHTNING-3B (1,833 events, 11 users, still firing on v2.16.6) groups four distinct call sites under one Sentry fingerprint. #4588 enumerates the REST API ones. The remaining two:

• /projects/:proj/w/<non-uuid> — LightningWeb.WorkflowLive.Collaborate.workflow_assigns/2 (collaborate.ex:372) calls Workflows.get_workflow!("null") when the path param is the literal string "null". The new-workflow branch only fires when id is absent, not when it's a non-UUID. Seen 9× in the recent 90-day Sentry sample. Frontend is generating /w/null somewhere — worth a separate ticket.
• Dataclips name-or-id filter — Lightning.Invocation.dataclip_where_filter/1 → id_prefix_interval/1 (invocation.ex:1037,1070) builds invalid UUID range bounds when users type non-hex text into the dataclips search box (e.g. IQ146-26-000-037). Seen 5× in the recent sample.

Implementation notes

Two patterns, drawn from common Phoenix practice:

1. LiveView on_mount UUID validator — equivalent of the controller-level validator from #4588 but for LiveView. A shared on_mount callback (sibling to LightningWeb.Hooks.:project_scope) runs Ecto.UUID.cast/1 on known id params and redirects to 404 (or the new-workflow route) on :error. Apply to WorkflowLive.Collaborate at minimum; audit other LiveViews mounting on :id path params.
   • Narrower alternative: workflow_assigns/2 treats a non-UUID id the same as a missing one. Simpler but doesn't generalize.
2. Filter input sanitization — id_prefix_interval/1 should reject non-hex input (or upstream, the LiveView assigning :name_or_id_part should validate before passing to dataclip_where_filter/1). Non-hex queries fall back to name-only search.

Whatever shared validate_uuid/1-style helper lands as part of #4588 should also be reachable from LiveView; worth coordinating so we don't end up with two implementations.

Release notes

Bug fix: invalid IDs in LiveView routes and dataclips search inputs no longer return 500.

User acceptance criteria

• Navigating to /projects/:p/w/null or /projects/:p/w/<non-uuid> does not crash — either 404 or renders the new-workflow view
• Dataclips search accepts arbitrary text (including dashes, non-hex chars) without 500
• Frontend /w/null generation tracked as a separate follow-up

Start with LightningWeb.WorkflowLive.Collaborate.workflow_assigns/2 in collaborate.ex:372 and Lightning.Invocation.dataclip_where_filter/1 with id_prefix_interval/1 in invocation.ex:1037,1070. Read issue #4588 and coordinate with its shared UUID helper before auditing other LiveViews with :id params. Done means invalid workflow IDs and arbitrary dataclips search text no longer produce 500 responses.