Stop the page Content tab from shadowing the content field (#4271)

[wakqasahmed]

Fixes #4271

In system/blueprints/pages/default.yaml, the Content tab and the markdown content field are both keyed content. BlueprintSchema flattens every field into one map keyed by name, containers included, and it does this by writing the field's own entry after recursing into its children, so the tab's entry gets written last and wins. The markdown field's validate config sits in that map under content too, but it's never the one that gets read back, so it never actually runs.

I renamed the tab's key to content_tab and left the field's key alone, since content is the actual page body key used throughout Grav and themes and isn't safe to touch. modular.yaml and external.yaml both extend default.yaml and override that same tab by key, so I updated them to content_tab too. Otherwise their overrides would stop merging into the tab and would themselves collide with the content field the same way.

I checked how much this actually changes at runtime before assuming a rename alone was safe. checkSafety() (the XSS scan) already runs on this field today regardless of which rule wins, since it only skips fields typed unset, so no new XSS behavior gets switched on here. Type validation is the part that starts working, and the field already has max: 0 set from #3643 to opt out of the length cap, so in practice this now only requires content to be a string, which is what a textarea always posts anyway.

Added a test that loads the real pages/default.yaml blueprint and asserts the flattened content item resolves to the markdown field's type and validate config rather than the tab's.

Ran the existing blueprint/data tests plus the full page test suite locally, all green, and confirmed the new test fails against the old blueprint (resolves to type tab) and passes with the fix.