Bump Tesla to 1.18.2 to fix mid-stream transport error crashes

[elias-ba]

Description

Upgrades Tesla from 1.15.3 to 1.18.2. The Finch adapter in 1.15.3 raises CaseClauseError on the 3-tuple error shape Finch returns when a streaming connection drops mid-response, which crashes the AI assistant worker. Tesla 1.16.0 (elixir-tesla/tesla#819) handles the shape gracefully so the worker can return a normal error; 1.18.2 is the latest stable on that line.

Closes #4781.

Validation steps

1. mix compile --warnings-as-errors succeeds.
2. Tesla-using suites pass: apollo_client, adaptor_registry, auth_providers, github_client, usage_tracking, ai_assistant, version_control.

Additional notes for the reviewer

AI Usage

• I have used Claude Code

Pre-submission checklist

• I have performed an AI review of my code
• I have implemented and tested all related authorization policies
• I have updated the changelog
• I have ticked a box in "AI usage" in this PR

[github-actions]

Security Review ✅

• S0 (project scoping): N/A — PR only bumps Tesla to 1.18.2 (CHANGELOG, mix.exs, mix.lock); no application code, queries, or web-layer entrypoints changed.
• S1 (authorization): N/A — no new controller actions, LiveView handlers, or policy modules touched.
• S2 (audit trail): N/A — no writes to project/instance configuration resources introduced.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.2%. Comparing base (79a9ecc) to head (46547e7).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4782     +/-   ##
=======================================
- Coverage   90.2%   90.2%   -0.0%     
=======================================
  Files        442     442             
  Lines      22439   22439             
=======================================
- Hits       20242   20236      -6     
- Misses      2197    2203      +6     

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