fix(ego-lint): check ALL schema IDs for settings-schema match

[ZviBaratz]

Problem

When gschema.xml contains multiple <schema> elements (e.g. a main schema plus a .keybindings sub-schema), the previous code extracted only the first <schema id=...> attribute. If a sub-schema appeared first — as in PaperWM, where org.gnome.shell.extensions.paperwm.keybindings precedes the main org.gnome.shell.extensions.paperwm schema — schema/id-matches and schema/filename-convention both produced false-positive FAILs even though the correct schema was present.

Root Cause

extract_schema_id() used | head -1, so it always returned the first <schema id=...> attribute regardless of order. The schema/id-matches check then compared that first ID against settings-schema, and the schema/filename-convention check expected the file to be named after that first ID.

Fix

• Add extract_all_schema_ids() — returns all <schema id=...> values from a file
• Refactor extract_schema_id() to use it (single-schema case is unchanged)
• Add schema_id_in_file(file, id) — returns 0 if the target ID is in any schema element
• schema/id-matches: pass if settings-schema is found in any <schema> element (not just the first)
• schema/filename-convention: when settings-schema is defined in metadata.json, use it as the reference ID for the expected filename (instead of the first schema found)

Test Coverage

• New multi-schema@test fixture reproduces the PaperWM layout: .keybindings sub-schema appears first, main schema second
• 4 new assertions: both checks produce PASS (not FAIL) on multi-schema XML

Affected Extensions

• PaperWM — primary case: 2 false-positive FAILs eliminated
• Any extension using a single gschema.xml with sub-schemas (keybindings, overrides, etc.)

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