fix: change external_run_id to string type in FlatRun proto

[AmanuelAaron]

Ticket

ET-649

Description

External Run Id should be a string in the SearchRuns response

Test Plan

Go to the Runs Table and filter by External Run ID & Tags.

Checklist

• Changes have been manually QA'd
• New features have been approved by the corresponding PM
• User-facing API changes have the "User-facing API Change" label
• Release notes have been added as a separate file under docs/release-notes/
  See Release Note for details.
• Licenses have been included for new code which was copied and/or modified from any external code

[netlify]

✅ Deploy Preview for determined-ui canceled.

Name	Link
🔨 Latest commit	01b79e9
🔍 Latest deploy log	https://app.netlify.com/sites/determined-ui/deploys/66a7de830e4c3a000920bbc4

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.12%. Comparing base (0df7ad3) to head (01b79e9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9744   +/-   ##
=======================================
  Coverage   54.12%   54.12%           
=======================================
  Files        1257     1257           
  Lines      154768   154768           
  Branches     3479     3477    -2     
=======================================
+ Hits        83762    83763    +1     
+ Misses      70860    70859    -1     
  Partials      146      146           

Flag	Coverage Δ
backend	44.87% <ø> (+<0.01%)	⬆️
harness	72.62% <ø> (ø)
web	53.39% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
webui/react/src/types.ts	99.70% <100.00%> (ø)

... and 4 files with indirect coverage changes