fix: resolve gosec lint errors in client.go

[k1LoW]

Summary

• Add defer for context.WithCancel cancel functions to fix G118 (cancel function not called) warnings
• Add //nolint:gosec for intentional OAuth token file encoding to suppress G117 (secret pattern match) false positive

[github-actions]

Code Metrics Report

	main (15b675a)	#502 (43a20ac)	+/-
Coverage	33.8%	33.7%	-0.1%
Code to Test Ratio	1:0.9	1:0.9	-0.1
Test Execution Time	11s	11s	0s

Details

  |                     | main (15b675a) | #502 (43a20ac) |  +/-  |
  |---------------------|----------------|----------------|-------|
- | Coverage            |          33.8% |          33.7% | -0.1% |
  |   Files             |             31 |             31 |     0 |
  |   Lines             |           3405 |           3407 |    +2 |
  |   Covered           |           1151 |           1151 |     0 |
- | Code to Test Ratio  |          1:0.9 |          1:0.9 |  -0.1 |
  |   Code              |           7020 |           7022 |    +2 |
  |   Test              |           6581 |           6581 |     0 |
  | Test Execution Time |            11s |            11s |    0s |

Code coverage of files in pull request scope (0.0% → 0.0%)

Files	Coverage	+/-	Status
client.go	0.0%	0.0%	modified

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Reported by octocov