Remove String.encodeBase64 from the codebase.

[zibet27]

Subsystem
Shared

Motivation
There are some usages of the deprecated String.encodeBase64 method that lead to warnings during tests and create noise.

[coderabbitai]

Walkthrough

Replaced usages of the custom encodeBase64() extension with kotlin.io.encoding.Base64.encode() across client, server, HTTP, websocket, and test modules; adjusted imports accordingly. Also removed a test-server makeString helper and dropped an internal logger in OAuth2 implementation.

Changes

Cohort / File(s)	Summary
Client auth & request tests ktor-client/ktor-client-apache/jvm/test/.../RequestProducerTest.kt, ktor-client/ktor-client-core/common/src/io/ktor/client/request/utils.kt, ktor-client/ktor-client-plugins/ktor-client-auth/common/src/io/ktor/client/plugins/auth/providers/BasicAuthProvider.kt, ktor-client/ktor-client-tests/common/src/io/ktor/client/tests/utils/Generators.kt	Replaced encodeBase64() uses with Base64.encode(...); added kotlin.io.encoding.Base64 imports and removed io.ktor.util.* imports; logic unchanged.
HTTP & WebSocket ktor-http/common/src/io/ktor/http/Cookie.kt, ktor-http/common/src/io/ktor/http/websocket/Utils.kt	Switched BASE64 encoding paths to Base64.encode(...) for cookie and websocket nonce handling; updated imports.
Server auth implementation & tests ktor-server/ktor-server-plugins/ktor-server-auth/common/src/io/ktor/server/auth/OAuth2.kt, ktor-server/ktor-server-plugins/ktor-server-auth/common/test/.../AuthBuildersTest.kt, .../BasicAuthTest.kt, .../CryptoTest.kt, .../OAuth2Test.kt	Replaced encodeBase64() with Base64.encode(...) in auth header construction and tests; removed an internal logger import/instance from OAuth2.
Server sessions & test utilities ktor-server/ktor-server-tests/common/test/io/ktor/tests/server/sessions/SessionTest.kt, ktor-test-server/src/main/kotlin/test/server/ServerUtils.kt	Session cookie encoding switched to Base64.encode(...); removed makeString(size: Int) helper and its io.ktor.util.* import.
Base64 tests ktor-utils/common/test/io/ktor/util/Base64Test.kt	Adjusted test encodings to use byte-array based encoding paths and added deprecation suppression where applicable.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• KTOR-5241 Deprecate Base64 encode/decode helpers, ensure strict decoding #5178: Replaces Ktor's custom encodeBase64() usage with the Kotlin standard library kotlin.io.encoding.Base64.encode(...) across modules (directly related).

Suggested reviewers

• osipxd
• bjhham

🚥 Pre-merge checks | ✅ 1 | ❌ 2

❌ Failed checks (1 warning, 1 inconclusive)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 10.34% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check	❓ Inconclusive	The description provides subsystem and motivation but omits the required Solution section describing how the deprecated method was replaced.	Add a Solution section explaining that String.encodeBase64 was replaced with kotlin.io.encoding.Base64.encode() calls throughout the codebase.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: removing deprecated String.encodeBase64 usage from the codebase.

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✨ Finishing touches

• 📝 Generate docstrings

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[bjhham]

Thank you 🙏

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