Apply supportsUnifiedCompletions to the default provider path

The completions/ghost-text suppression driven by supportsUnifiedCompletions
was only wired into the joint completions provider, which is gated behind
InlineEditsJointCompletionsProviderEnabled (default false). On the default
path, InlineEditProviderFeature registers NES and suppresses completions via
the modelUnification-derived excludes, so ghost text kept firing when only
the prompting strategy was set.

Derive the effective unification in InlineEditProviderFeature from the baked
supportsUnifiedCompletions flag (OR-ed with the core modelUnification state),
so both 'completions' and 'github.copilot' are excluded purely from the
selected model's strategy, matching the excludedProviders behavior.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 676b2f6f-4b80-4754-8dde-708d7e992148