`resolve()` raises an error instead of returning an empty collection for non-URI inputs

[luisfabib]

Resolve.evaluate() raises FHIRPathError when the input item is not a string URI or a
Reference object. The FHIRPath spec states that items that cannot be resolved must be
silently skipped (i.e. contribute an empty result), not raise an error.

Additionally, external http:// / https:// URL resolution is completely absent. A warning
is logged but no attempt is made. The spec does allow engines to return empty for external
references when no resolution service is configured, but this should be a documented opt-out,
not a hard block.

Expected behaviour (per spec)

• Non-URI, non-Reference inputs: silently produce no output (empty collection).
• External URLs with no resolution service configured: return empty collection (no error, no warning spam).
• External URLs with a resolution service configured: delegate to the service.

Actual behaviour

• Non-URI inputs: raises FHIRPathError.
• External URLs: emits a warnings.warn() and still returns [] — the warn adds noise in
  production validation runs where resolve() is common.

Suggested fix

Replace the raise FHIRPathError(...) branch for non-URI items with a continue (skip).
Tie external URL resolution to the proposed TerminologyService/ResolutionService interface
so it can be opted into.