LIN-1956 Document custom channel campaign APIs

[tashviks]

Summary

• Documents custom_channel_names on the Create Campaign API for project-level custom channel grouping.
• Adds the current one-custom-channel-per-campaign limit and reserved/default channel-name restrictions.
• Documents the campaign response custom_channels field as canonical while noting legacy alias fields may appear.
• Updates Edit Campaign docs to show setting/replacing a custom channel and clearing it with custom_channel_names: [].
• Corrects the relative campaign edit/delete endpoint examples and the JavaScript edit method to PATCH.
• Adds custom-channel-specific validation errors, including the inactive-channel conflict response.

Validation

• git diff --check -- api-reference/campaign-apis.mdx
• npm run build attempted, but the installed Mintlify CLI does not expose a build command (Unknown command: build).
• npx mintlify broken-links attempted; it still reports existing repo-wide broken links outside this changed page, mostly under ad-networks/tiktok.mdx, features/*, and SDK pages.

Notes

• Branch is cut from latest origin/main.
• This PR only changes api-reference/campaign-apis.mdx.