feat(validation): reject empty/whitespace-only channel names — closes #479

[kitWarse]

Closes #479

Changes

• �atch-validator.ts — exported �alidateChannelName() with EMPTY_CHANNEL_NAME error code for null/empty/whitespace-only channel names; BatchValidator now routes through it
• �atch-validator.test.ts — unit tests for empty/whitespace-only channel rejection + full �alidateChannelName describe block
• API_ERROR_REFERENCE.md — added EMPTY_CHANNEL_NAME to the batch validation error codes table

Acceptance Criteria

Empty names are rejected	'' → EMPTY_CHANNEL_NAME
Whitespace-only rejected	' ' → EMPTY_CHANNEL_NAME
Validation is documented	API_ERROR_REFERENCE.md updated
Unit tests are added	alidateChannelName describe block + BatchValidator test

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