[Fusion] Reject empty authorization groups in source schemas

[michaelstaib]

Ticket: hc-fork-2-hdw.5 (Reject empty authorization groups in source schemas)

Stacked on #26. This PR's diff is against mst/auth-federation-intake.

An empty AND group is satisfied by anyone, so it would let every caller through. AuthorizationDirectiveArgumentRule now rejects these with INVALID_AUTHORIZATION_DIRECTIVE_ARGUMENT:

• an empty outer list,
• an empty inner group,
• an empty or whitespace-only string.

This covers @requiresScopes and @policy on fields and types, and Apollo intake hits the same rule. AuthorizationGroups.Reduce no longer skips empty factors.

Also carried from the #26 review:

• A test for a truly incompatible own declaration (a different argument name).
• The compatible-declaration test is renamed to match what it checks.

Verified:

• dotnet build src/All.slnx: 0 errors.
• Fusion.Composition.Tests: 3448/3448.
• Fusion.Composition.ApolloFederation.Tests: 240/240.
• Nitro CLI Fusion command tests: 1928/1928.

Minor review notes, not fixed:

• INVALID_AUTHORIZATION_DIRECTIVE_ARGUMENT is an error but is listed in the Warnings table of composition.md. That came from [Fusion] Merge the source authorization directives into fusion__authorization #25.
• No test covers a blank string as an outer-list item.
• ParseGroups has no fallback guard if the rule is ever bypassed.
• Prettier was not run on composition.md. Table padding was checked with a script.

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