test: migrate durable task TLS tests

[NikStack20]

Description

Migrates the commented-out Durable Task gRPC TLS test to the current Java SDK test pattern.

Changes

• Replaced the legacy certificate-generation based test implementation.
• Added pre-generated TLS test certificates as test resources.
• Added coverage for TLS, CA certificates, mTLS, invalid TLS configuration, insecure TLS, and plaintext connections.
• Ensured the Durable Task client is closed after each test.

Validation

• DurableTaskGrpcClientTlsTest: 7 tests passed.
• durabletask-client test suite: 94 tests passed.
• git diff --check: clean.

Related to #1604.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.08%. Comparing base (4192ca9) to head (68d6923).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1802      +/-   ##
============================================
- Coverage     77.15%   77.08%   -0.07%     
+ Complexity     2317     2316       -1     
============================================
  Files           245      245              
  Lines          7186     7186              
  Branches        750      750              
============================================
- Hits           5544     5539       -5     
- Misses         1281     1285       +4     
- Partials        361      362       +1     

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