Add Pythonic SageMaker training and processing

[rohitrsh]

Stacked PR 2/2 — do not merge out of order.

This branch currently includes the commit from #3456. After #3456 merges, this branch will be rebased onto master so the final review contains only the Pythonic SageMaker change.

Summary

• Adds Pythonic SageMaker Training and Processing tasks that execute ordinary Flyte @task functions inside SageMaker-managed containers.
• Introduces shared entrypoint, environment, job naming, output, error, retry, and cancellation handling for Pythonic SageMaker jobs.
• Requires an explicit container image and region, limits Pythonic jobs to one instance, validates SageMaker entrypoint limits, and rejects incompatible Processing network isolation.
• Documents image requirements, IAM behavior, model artifacts, inputs/outputs, and the distinction between declarative and Pythonic modes.
• Treats RETRYABLE_FAILED as terminal for local connector execution and adds regression coverage.

Stack order:

1. Add managed SageMaker job connectors #3456 — Managed SageMaker job connectors
2. Pythonic SageMaker Training and Processing (this PR)

Test plan

• Focused Pythonic and managed Training/Processing connector suite — 74 passed
• Core connector utility suite — 15 passed
• SageMaker plugin lint and formatting checks
• Plugin packaging, serialization, and import smoke tests
• DCO, confidentiality, attribution, and internal-reference audits

Merge only after #3456. Before merge, rebase this branch onto upstream master, rerun all local and hosted verification gates, and mark the PR ready for review.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 47.79%. Comparing base (c6c20a1) to head (6cf52df).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
flytekit/extend/backend/utils.py	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (c6c20a1) and HEAD (6cf52df). Click for more details.

HEAD has 26 uploads less than BASE

Flag	BASE (c6c20a1)	HEAD (6cf52df)
	27	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3459       +/-   ##
===========================================
- Coverage   80.15%   47.79%   -32.37%     
===========================================
  Files         269      217       -52     
  Lines       25184    22943     -2241     
  Branches     3036     3036               
===========================================
- Hits        20187    10966     -9221     
- Misses       4177    11370     +7193     
+ Partials      820      607      -213     

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