(v1-0/3) tea.go: all fixes combined (for convenience/CI)

[desertwitch]

Here's all my deadlock/race condition/error handling fixes stacked and combined for review convenience and CI-testing.

Includes:

• (v1-1/3) tea.go: fix deadlock on context cancellation #1372
• (v1-2/3) tea.go: teardown-related deadlock & race condition fixes #1373
• (v1-3/3) tea.go: fix panic handling race condition + wrongful nil returns despite errors (ctx, panic, ...) #1375

Many thanks for your efforts and bearing with my pull requests. 💯 🚀

For a v2-targeted version of these fixes see:

• (v2) tea.go: deadlock/error return fixes, additional tests (targeted at v2) #1388

[codecov]

Codecov Report

Attention: Patch coverage is 98.41270% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (main@6a1ebaa). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
tea.go	98.38%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1376   +/-   ##
=======================================
  Coverage        ?   70.10%           
=======================================
  Files           ?       17           
  Lines           ?     1726           
  Branches        ?        0           
=======================================
  Hits            ?     1210           
  Misses          ?      469           
  Partials        ?       47           

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