merge source meta and table meta on SourceDefinition.meta

[MichelleArk]

resolves #9766

Problem

SourceDefinition.meta only includes table-level meta, which means it is not usable via the RelationConfig protocol to access source-level meta.

Solution

• Merge source-level and table-level meta into SourceDefinition.meta, with table-level taking precedence
• Preserve SourceDefinition.source_meta to keep this change backward-compatible

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.13%. Comparing base (c501d71) to head (cf393cd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9723      +/-   ##
==========================================
+ Coverage   88.11%   88.13%   +0.01%     
==========================================
  Files         178      178              
  Lines       22479    22480       +1     
==========================================
+ Hits        19807    19812       +5     
+ Misses       2672     2668       -4     

Flag	Coverage Δ
integration	85.51% <100.00%> (+0.01%)	⬆️
unit	62.74% <100.00%> (+0.19%)	⬆️

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