chore: Update poetry instructions

[devin-ai-integration]

chore: Add missing make target; update poetry instructions

This PR applies the same changes made to CONTRIBUTING.md in python-server-sdk PR #338 to maintain consistency across LaunchDarkly Python repositories.

Changes Made

• Updated poetry setup instructions to be more accurate and user-friendly
• Changed "active the poetry shell" to "ensure your dependencies are installed and (optionally) activate the virtualenv"
• Reordered poetry commands to use poetry install followed by eval $(poetry env activate) instead of poetry shell

Rationale

The original poetry shell command can sometimes cause issues in certain environments. The updated approach using eval $(poetry env activate) provides a more reliable way to activate the poetry virtual environment while making the activation step optional for developers who prefer to manage their environment differently.

Testing Transparency

What I Actually Checked

• ✅ Verified the exact changes match those from the original python-server-sdk PR #338
• ✅ Confirmed the poetry setup section exists in this repository's CONTRIBUTING.md
• ✅ Applied identical text changes to maintain consistency across repositories
• ✅ Verified the branch targets mk/sdk-1350/setup-fixes as requested

What I Did Not Check

• ❌ Did not test the actual poetry commands in this repository's environment
• ❌ Did not verify if this repository has any unique poetry configuration requirements
• ❌ Did not run any local build or test commands to verify the instructions work

Reviewer Checklist

• Verify the poetry setup instructions are appropriate for this repository
• Test that the updated commands work correctly in a fresh environment
• Confirm no other documentation needs similar updates

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Link to Devin run: https://app.devin.ai/sessions/53474e9243c3429fa6fad0798fa5b5bd

Requested by: mkeeler@launchdarkly.com

[devin-ai-integration]

Original prompt from mkeeler@launchdarkly.com:

SYSTEM:
=== BEGIN THREAD HISTORY (in #sdks-python) ===
GitHub (U01V5FFUQ1E): 

[launchdarkly/python-server-sdk] Pull request opened by keelerm84

Matthew Keeler (U02FZSQUVCL): Hey @Devin, check out the changes I made in this PR to the CONTRIBUTING.md file. I need you to make those same changes to:

1. <https://github.com/launchdarkly/python-server-sdk-otel/blob/main/CONTRIBUTING.md> 
2. <https://github.com/launchdarkly/openfeature-python-server/blob/main/CONTRIBUTING.md> 
3. <https://github.com/launchdarkly/python-server-sdk-ai/blob/main/CONTRIBUTING.md>
4. <https://github.com/launchdarkly/python-eventsource/blob/main/CONTRIBUTING.md>
For each, you can make the PR against the branch mk/sdk-1350/setup-fixes

Matthew Keeler (U02FZSQUVCL): @Devin see above
=== END THREAD HISTORY ===

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[devin-ai-integration]

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