Set IsDefault to False for echo plugin

[pingsutw]

Tracking issue

NA

Why are the changes needed?

If IsDefault is True, the unknown task will always be handled by the echo plugin and always succeed.

{"json":{"exec_id":"fd2ed7347cfdb4e338f2","node":"n0","ns":"kk-flyte-dev3","res_ver":"9901776719","routine":"worker-
0","src":"handler.go:340","tasktype":"gnn","wf":"shuliang-
projects:development:gnn_agent_test.wf"},"level":"warning","msg":"No plugin found for Handler-type [gnn], defaulting to 
[echo]","ts":"2024-08-31T05:51:07Z"}

What changes were proposed in this pull request?

Set IsDefault to false

How was this patch tested?

single binary

Setup process

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

Docs link

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36.16%. Comparing base (7a91799) to head (38f0380).
Report is 153 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5713      +/-   ##
==========================================
- Coverage   36.17%   36.16%   -0.01%     
==========================================
  Files        1303     1303              
  Lines      109663   109663              
==========================================
- Hits        39667    39660       -7     
- Misses      65851    65858       +7     
  Partials     4145     4145              

Flag	Coverage Δ
unittests-datacatalog	51.37% <ø> (ø)
unittests-flyteadmin	55.25% <ø> (-0.04%)	⬇️
unittests-flytecopilot	12.17% <ø> (ø)
unittests-flytectl	62.18% <ø> (ø)
unittests-flyteidl	7.12% <ø> (ø)
unittests-flyteplugins	53.34% <ø> (ø)
unittests-flytepropeller	41.76% <ø> (ø)
unittests-flytestdlib	55.33% <ø> (-0.03%)	⬇️

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[Future-Outlier]

I've checked the code, let's merge it.