Add typed clients for the seven uncovered Agent Server operations

Objective

Resolve the seven actionable server-only operations reported by the pinned Agent Server endpoint audit. This issue intentionally excludes documented non-divergences such as operational endpoints omitted from the filtered public artifact, client-ahead meta-profile APIs, and workspace routes already exposed as authenticated browser URLs.

Audit context: https://github.com/OpenHands/typescript-client/pull/307#issuecomment-5105956958

Uncovered operations

• GET /api/conversations/{conversation_id}/events
• GET /api/file/archive
• GET /api/git/commits
• GET /api/git/commits/{sha}/changes
• GET /api/init
• POST /api/conversations/{conversation_id}/load_plugin
• POST /api/init

Implementation guidance

• Use stable aliases derived from the generated Agent Server v1.38 contract; do not introduce duplicate handwritten request/response models.
• Keep public source code browser-compatible.
• Add focused clients and type-level/runtime tests for each operation.
• Reuse or simplify existing work where appropriate:
  • OpenHands/typescript-client#246 covers batch conversation events and can now be reduced because its supporting transport work has landed.
  • OpenHands/typescript-client#243 covers deferred initialization and should be updated to use generated aliases.
• Add typed git commit/change methods and migrate existing low-level consumers where practical.
• Model archive download with an explicit browser-compatible binary return type.
• Add a typed conversation plugin-loading method.
• Separate implementation PRs are acceptable when the operations have different owners or semantics.

Exit criteria

• The endpoint audit reports zero actionable server-only operations.
• Generated contract regeneration remains clean.
• Handwritten signatures are statically checked against generated operations.
• Unit, build, and relevant integration tests pass.