fix(taps): Avoid setting up mapper in discovery mode

[edgarrmondragon]

Closes #1821

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📚 Documentation preview 📚: https://meltano-sdk--1835.org.readthedocs.build/en/1835/

[codecov]

Codecov Report

Merging #1835 (75372fe) into main (d372080) will increase coverage by 0.04%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main    #1835      +/-   ##
==========================================
+ Coverage   86.48%   86.53%   +0.04%     
==========================================
  Files          59       59              
  Lines        4974     4990      +16     
  Branches      808      811       +3     
==========================================
+ Hits         4302     4318      +16     
+ Misses        477      476       -1     
- Partials      195      196       +1     

Impacted Files	Coverage Δ
singer_sdk/target_base.py	92.09% <66.66%> (-0.47%)	⬇️
singer_sdk/tap_base.py	80.80% <83.33%> (-0.28%)	⬇️
singer_sdk/plugin_base.py	86.22% <100.00%> (+2.15%)	⬆️

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[pnadolny13]

@edgarrmondragon I'm not sure I understand the bug completely but it sounds like you were able to replicate the error and I see the main change is that you're not setting up the mapper when doing discovery. Looks good to me!