Fix spurious @notice error for untagged docs on file-level variables

[heathdutton]

Fixes #16355

Untagged documentation comments on file-level variables (like /** whatever... */ above a constant) were being implicitly converted to @notice, which then failed validation since @notice is not valid for file-level variables. Users would get an error for a tag they never explicitly used.

This fix passes the set of valid tags to the DocString parser so it can check whether @notice is valid before adding it implicitly. When it's not valid, the parser skips the untagged text and continues processing so any explicit valid tags like @dev still work.

Changes:

• Pass valid tags to DocStringParser
• Skip implicit @notice when not valid for the node type
• Add syntax tests for the new behavior

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