fix(talk_app): Ignore invalid text selection when searching mentions

[provokateurin]

Extracted from #2647

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.89%. Comparing base (1e4928e) to head (34cb22f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2700   +/-   ##
=======================================
  Coverage   28.89%   28.89%           
=======================================
  Files         373      373           
  Lines      136630   136634    +4     
=======================================
+ Hits        39481    39485    +4     
  Misses      97149    97149           

Flag	Coverage Δ		*Carryforward flag
account_repository	98.47% <ø> (ø)
cookie_store	99.48% <ø> (ø)		Carriedforward from 1e4928e
dashboard_app	96.05% <ø> (ø)
dynamite	31.05% <ø> (ø)		Carriedforward from 1e4928e
dynamite_end_to_end_test	61.69% <ø> (ø)		Carriedforward from 1e4928e
dynamite_runtime	85.40% <ø> (ø)		Carriedforward from 1e4928e
interceptor_http_client	97.18% <ø> (ø)		Carriedforward from 1e4928e
neon_dashboard	96.05% <ø> (ø)		Carriedforward from 1e4928e
neon_framework	59.25% <ø> (ø)
neon_http_client	93.61% <ø> (ø)		Carriedforward from 1e4928e
neon_notifications	100.00% <ø> (ø)		Carriedforward from 1e4928e
neon_rich_text	100.00% <ø> (ø)
neon_storage	94.66% <ø> (ø)		Carriedforward from 1e4928e
neon_talk	99.45% <ø> (ø)		Carriedforward from 1e4928e
nextcloud	24.33% <ø> (ø)		Carriedforward from 1e4928e
notifications_app	97.43% <ø> (ø)
notifications_push_repository	98.11% <ø> (ø)
sort_box	90.90% <ø> (ø)		Carriedforward from 1e4928e
talk_app	98.82% <100.00%> (+<0.01%)	⬆️

*This pull request uses carry forward flags. Click here to find out more.

Files with missing lines	Coverage Δ
...ckages/talk_app/lib/src/widgets/message_input.dart	98.60% <100.00%> (+0.04%)	⬆️

[Leptopoda]

Can we add a test for this?

[provokateurin]

I don't remember how I triggered this, so I don't know how to add a test for it 😅