Warn on invalid markdown snippet markers

[ahfoysal]

Refs #501

This warns when registry update-markdown sees an HTML comment that looks like a semconv/weaver snippet marker but does not parse as one. It also requires whitespace after semconv and weaver marker keywords so typos like <!-- semconvmetric.foo --> are not treated as valid snippets.

Tests:

• cargo fmt --all --check
• cargo test -p weaver_semconv_gen recognizes_invalid_snippet_markers
• cargo test -p weaver test_registry_update_markdown_warns_on_invalid_snippet_marker

[linux-foundation-easycla]

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