fix(structlog): preserve existing processors

[hsusul]

Description

Preserve an application-configured StructlogProcessor when instrumenting and uninstrumenting Structlog.

Previously, instrumentation could add a duplicate processor when one was already configured. Uninstrumentation then removed processors by type, which could remove the application-owned processor as well.

The instrumentor now avoids duplicate insertion and removes only the exact processor instance it created.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change
• This change requires a documentation update

How Has This Been Tested?

• Oldest supported Structlog package test environment
• Latest supported Structlog package test environment
• Focused regression tests covering duplicate insertion and ownership during uninstrumentation
• Structlog package lint
• Ruff and Ruff formatting checks
• License-header check

The root uv sync --frozen --all-packages command could not complete because the local system lacks the MySQL/MariaDB development libraries required to build mysqlclient. This does not affect the Structlog package test environments.

Does This PR Require a Core Repo Change?

• Yes
• No

Checklist:

• Followed the style guidelines of this project
• Changelog fragment will be added after the PR number is assigned
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

• ❌ - login: @hsusul / name: Henry Su. The commit (d69d95e) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please visit our EasyCLA portal and chat with our support bot.