feat: add single Codeflare entrypoint with handler pattern and remove legacy auth

[szaher]

Summary

• Introduces Codeflare class as the single SDK entrypoint — authentication,
  namespace, retries, timeout, and logging configured once via SDKConfig
• Adds cf.clusters.* and cf.jobs.* handler pattern (thin orchestrators
  that delegate to existing Cluster/RayJob classes)
• Removes legacy auth classes (TokenAuthentication, KubeConfigFileAuthentication,
  Authentication, KubeConfiguration) — kube-authkit is now the only auth method
• Updates all demo notebooks and E2E tests to use the new entrypoint

Usage

from codeflare_sdk import Codeflare, SDKConfig

cf = Codeflare(config=SDKConfig(
    auth=AuthConfig(method="auto"),
    namespace="my-project",
))

cluster = cf.clusters.create(name="train", num_workers=4)
cluster.apply()
job = cf.jobs.submit(name="train-job", entrypoint="python train.py")

Test plan

- 394 unit tests pass (32 new), 2 skipped
- 100% coverage on codeflare.py, 93% on auth.py
- Pre-commit (black, trailing whitespace, YAML) passes
- E2E tests updated — verify on OpenShift cluster
- Demo notebook 6_single_entrypoint.ipynb runs end-to-end on a real cluster

# What changes have been made
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# Verification steps
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## Checks
- [ ] I've made sure the tests are passing. 
- Testing Strategy
 - [ ] Unit tests
 - [ ] Manual tests
 - [ ] Testing is not required for this change

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[openshift-ci]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.68%. Comparing base (ce6b6f6) to head (7fc63d9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1091      +/-   ##
==========================================
+ Coverage   96.61%   96.68%   +0.07%     
==========================================
  Files          23       24       +1     
  Lines        2306     2296      -10     
==========================================
- Hits         2228     2220       -8     
+ Misses         78       76       -2     

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