test(agent-service): raise unit-test coverage of core modules

[bobbai00]

What changes were proposed in this PR?

The agent service had low automated test coverage (~52% line / ~54% function), concentrated in its core logic. This PR adds unit tests for the previously under-tested modules:

Area	New test file(s)
API clients (mocked fetch, positive + non-ok + malformed)	api/workflow-api.test.ts, api/compile-api.test.ts, api/backend-api.test.ts
JWT decode / expiry / auth headers	api/auth-api.test.ts
System prompt builder (operator filtering, UDF guides)	agent/prompts.test.ts
Context assembly (completed/ongoing tasks, DAG, results)	agent/util/context-utils.test.ts
Operator materialization & input-schema resolution	agent/util/workflow-utils.test.ts
Workflow CRUD tools (add/modify/delete, error paths)	agent/tools/workflow-crud-tools.test.ts
Workflow execution tool (validation, run, formatting, failures)	agent/tools/workflow-execution-tools.test.ts
TexeraAgent in-memory surface (state, settings, tools, lifecycle)	agent/texera-agent.test.ts

A shared operator-metadata fixture (agent/util/metadata-fixture.ts) lets tests run without a live backend. No production code was changed.

Coverage (bun test --coverage): line 51.6% → 88.5%, function 54.2% → 90.0%.

Any related issues, documentation, discussions?

Closes #5266

How was this PR tested?

cd agent-service
bun test            # 165 pass, 0 fail, 372 expect() calls
bun run typecheck   # tsc --noEmit, clean
bun run format:check

Both positive and negative paths are covered (invalid input, missing config, non-ok HTTP responses, malformed responses).

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

Generated-by: Claude Opus 4.8 (1M context)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.05%. Comparing base (7bd6550) to head (3103719).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5274      +/-   ##
============================================
+ Coverage     49.01%   52.05%   +3.03%     
  Complexity     2378     2378              
============================================
  Files          1050     1051       +1     
  Lines         40336    40173     -163     
  Branches       4277     4277              
============================================
+ Hits          19772    20911    +1139     
+ Misses        19407    18105    -1302     
  Partials       1157     1157              

Flag	Coverage Δ		*Carryforward flag
access-control-service	39.53% <ø> (ø)		Carriedforward from 7bd6550
agent-service	71.80% <100.00%> (+38.03%)	⬆️
amber	51.58% <ø> (ø)		Carriedforward from 7bd6550
computing-unit-managing-service	0.00% <ø> (ø)		Carriedforward from 7bd6550
config-service	0.00% <ø> (ø)		Carriedforward from 7bd6550
file-service	37.99% <ø> (ø)		Carriedforward from 7bd6550
frontend	40.82% <ø> (ø)		Carriedforward from 7bd6550
python	90.79% <ø> (ø)		Carriedforward from 7bd6550
workflow-compiling-service	56.81% <ø> (ø)		Carriedforward from 7bd6550

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

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