docs: document user-agent template behavior with platform headers

[devin-ai-integration]

Summary

SDKs can send a custom User-Agent built from a user-agent template in generators.yml, and they can also send a structured User-Agent carrying OS, architecture, and runtime info when platform headers are enabled. Until recently, Python, TypeScript, and C# dropped the configured template when platform headers were on and fell back to the package name. That's now fixed, so customers who set a custom User-Agent for attribution or analytics get their exact product token in every request regardless of platform headers, and server-side telemetry is consistent across languages.

Implements docs for: fix(python, typescript, csharp): honor user-agent template with platform headers (fern-api/fern#17298)

Changes:

• Added a user-agent config option to the Python, TypeScript, C#, Go, PHP, and Ruby generator configuration references (previously only documented for Java), covering the placeholders and the resulting header.
• Documented the combined result on those pages: the resolved template becomes the product token, followed by the platform and runtime segments (plantstore-python-sdk/0.1.0 (linux; x86_64) Python/3.11.0), and a resolved value whose trailing segment isn't a version (@plantstore/sdk) is used as-is with no version appended.
• Cross-linked the includePlatformHeaders option on each page to the user-agent option.
• Added a note to the generators.yml reference config section that user-agent is CLI-resolved and behaves identically in every generator's config.

user-agent isn't a per-generator option: the CLI resolves it (CLI_ONLY_CONFIG_KEYS in checkVersionExists.ts) and threads the interpolated value through ir.sdkConfig.platformHeaders.userAgent, so it applies to every generator that reads that field. Rust and Swift are deliberately excluded: ClientConfigGenerator hardcodes "<ApiName> Rust SDK" and the Swift generator emits no User-Agent, so neither consumes the template. The reference says so explicitly.

Refresh against main

Main has since merged allowUserAgentAppInfo docs on all seven generator pages (#6547), plus runtime_version for Python. Re-verified against fern-api/fern main: the CLI-resolved template still lands in platformHeaders.userAgent and each generator's app-info token is appended after whatever User-Agent the SDK would otherwise send, so the claims in this PR still hold. Merged main and added the missing cross-links:

• Each user-agent section now notes that the app-info product token is appended after the resolved value, linking to that page's allowUserAgentAppInfo / allow-user-agent-app-info / allow_user_agent_app_info anchor.
• Python's section also notes that runtime_version replaces the resolved value's trailing version segment at runtime (_get_user_agent_coordinate_prefix in client_wrapper_generator.py applies to the template-resolved value, not just the package coordinate).

Validation: pre-commit run --files <changed files> passes (only pre-existing Vale findings) and fern check --warnings reports 0 errors.

Link to Devin session: https://app.devin.ai/sessions/2fdc861910a3458cbab59174b4d87b8c

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1785517757-user-agent-platform-headers.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/sdks/generators/csharp/configuration
• learn/sdks/generators/go/configuration
• learn/sdks/generators/java/configuration
• learn/sdks/generators/php/configuration
• learn/sdks/generators/python/configuration
• learn/sdks/generators/ruby/configuration
• learn/sdks/generators/typescript/configuration