[AMORO-3885] fix missing oss.endpoint for internal mixed_iceberg catalog

[liaoyt]

Why are the changes needed?

Close #3885.

Brief change log

How was this patch tested?

• Add some test cases that check the changes thoroughly including negative and positive cases if possible

• Add screenshots for manual tests if appropriate

• Run test locally before making a pull request

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 22.12%. Comparing base (17b68d0) to head (8a59f85).
⚠️ Report is 10 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (17b68d0) and HEAD (8a59f85). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (17b68d0)	HEAD (8a59f85)
core	1	0

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3886      +/-   ##
============================================
- Coverage     28.82%   22.12%   -6.70%     
+ Complexity     3838     2461    -1377     
============================================
  Files           625      445     -180     
  Lines         50514    40896    -9618     
  Branches       6455     5767     -688     
============================================
- Hits          14560     9050    -5510     
+ Misses        34928    31088    -3840     
+ Partials       1026      758     -268     

Flag	Coverage Δ
core	?
trino	22.12% <ø> (?)

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[zhoujinsong]

Thanks for the contribution!
I left a comment, PTAL.