other(tests): tests for bucketing manager

[huijjj]

🚀 Summary of Changes

Tests for bucketing manager codes

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📌 Related Issues / Tickets

• Resolves #
• Related to #

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✅ Type of Change

• 🚀 Release (release)
• ✨ Feature (feature)
• 🧠 Model support (model)
• 🧬 Core engine changes (core)
• 🛠 Bug fix (fix)
• ⚙️ Performance improvement (perf)
• 🔁 Refactor or code cleanup (refactor)
• 📄 Documentation (docs)
• ❓ Other (other): please describe

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🧪 How to Test

1. Run ...
2. Verify output: ...
3. Edge case tested: ...

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📸 Screenshots / Logs (if applicable)

Before

vllm_rbln/v1/worker/bucketing/__init__.py                                             15      9      8      0    26%   43-65
vllm_rbln/v1/worker/bucketing/bucketing_manager.py                                    34     20     12      0    30%   25-32, 37, 42, 47, 52, 56-59, 73-84
vllm_rbln/v1/worker/bucketing/exponential_bucketing_manager.py                        18     14      6      0    17%   28-37, 41-53
vllm_rbln/v1/worker/bucketing/linear_bucketing_manager.py                             16     12      4      0    20%   28-37, 41-47
vllm_rbln/v1/worker/bucketing/manual_bucketing_manager.py                             11      7      2      0    31%   26-28, 34-42

After

vllm_rbln/v1/worker/bucketing/__init__.py                                             15      0      8      0   100%
vllm_rbln/v1/worker/bucketing/bucketing_manager.py                                    34      0     12      0   100%
vllm_rbln/v1/worker/bucketing/exponential_bucketing_manager.py                        18      0      6      0   100%
vllm_rbln/v1/worker/bucketing/linear_bucketing_manager.py                             16      0      4      0   100%
vllm_rbln/v1/worker/bucketing/manual_bucketing_manager.py                             11      0      2      0   100%

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📋 Checklist

• PR title follows Conventional Commits format
• This PR is linked to an existing issue
• The test method is described, and the expected result is clearly stated
• Relevant documentation has been updated (if applicable)

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💬 Notes

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