fix(adk): copy-on-write SetMessageID to avoid concurrent map panic

[hi-pender]

Summary

• ChatModel streaming fans out the same *schema.Message pointers to multiple consumers inside the model body. Stamping the eino message ID into Extra via an in-place map write then races with concurrent reads of that shared map, triggering a fatal concurrent map read and map write panic.
• Make internal.SetMessageID copy-on-write: allocate a fresh map, copy existing entries, then set the ID. All callers already reassign the returned map (v.Extra = SetMessageID(...)), so the change is transparent and fixes every path at once.
• This removes the map-level panic. The field-level race on the exported Extra field cannot be eliminated by the framework while Extra is public, and is documented as a known limit in the function doc.

Test plan

• go build ./...
• go test -race ./adk/... (all green, incl. 30s race run on the adk root package)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.16%. Comparing base (f683210) to head (b9279a6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1085   +/-   ##
=======================================
  Coverage   83.15%   83.16%           
=======================================
  Files         162      162           
  Lines       23133    23134    +1     
=======================================
+ Hits        19237    19239    +2     
+ Misses       2632     2631    -1     
  Partials     1264     1264           

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