fix(agent-service): handle falsy model throws

[anthonychengit]

What changes were proposed in this PR?

The model adapter can throw arbitrary JavaScript values, including null and undefined. The previous error path immediately read error.name, so those values caused sendMessage itself to reject instead of returning Texera's normal error result.

Before: falsy model throw -> error.name dereference -> rejected sendMessage
After:  falsy model throw -> normalized error text -> resolved ReAct error step

This change safely normalizes thrown values and uses optional access for optional error metadata. The regression test covers null, undefined, false, 0, and the empty string, and verifies both the resolved response and its recorded error step.

Any related issues, documentation, discussions?

Closes #7485

How was this PR tested?

npx --yes bun@1.3.14 test src/agent/texera-agent.spec.ts
npx --yes bun@1.3.14 run typecheck
npx --yes bun@1.3.14 run format:check

The test file passed 56 tests. Type checking and formatting checks also passed.

Was this PR authored or co-authored using generative AI tooling?

Generated-by: OpenAI Codex (GPT-5)

[github-actions]

Automated Reviewer Suggestions

Based on the git blame history of the changed files, we recommend the following reviewers:

• Contributors with relevant context: @aglinxinyuan, @bobbai00
  You can notify them by mentioning @aglinxinyuan, @bobbai00 in a comment.

[github-actions]

Backport auto-label report

This fix: PR was checked against each actively-supported release branch. release/* labels drive the post-merge backport, so add or remove one to change where this fix lands.

Release branch	Analysis
✅ release/v1.2	Change detected on this branch — label added; this fix is queued to backport here. Requested review from @xuang7.

Auto-label run.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.65%. Comparing base (12169c2) to head (d6cf112).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #7638   +/-   ##
=========================================
  Coverage     89.65%   89.65%           
  Complexity     4397     4397           
=========================================
  Files          1177     1177           
  Lines         46996    46997    +1     
  Branches       5268     5268           
=========================================
+ Hits          42136    42137    +1     
  Misses         3094     3094           
  Partials       1766     1766           

Flag	Coverage Δ		*Carryforward flag
access-control-service	70.00% <ø> (ø)		Carriedforward from 12169c2
agent-service	98.62% <100.00%> (+<0.01%)	⬆️
amber	86.35% <ø> (ø)		Carriedforward from 12169c2
computing-unit-managing-service	72.46% <ø> (ø)		Carriedforward from 12169c2
config-service	77.31% <ø> (ø)		Carriedforward from 12169c2
file-service	68.90% <ø> (ø)		Carriedforward from 12169c2
frontend	90.67% <ø> (ø)		Carriedforward from 12169c2
notebook-migration-service	78.89% <ø> (ø)		Carriedforward from 12169c2
pyamber	97.57% <ø> (ø)		Carriedforward from 12169c2
workflow-compiling-service	57.89% <ø> (ø)		Carriedforward from 12169c2

*This pull request uses carry forward flags. Click here to find out more.

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