Add application-scoped state API (ctx.app_state)

[diptanu]

Summary

Some agents require updating context about users or objects they are managing. Application State is a way for applications to share state across request boundaries as they learn about the problem domain.

For example, a fitness tracker agent managing fitness logs of multiple users can store per-user context in app_state as a flexible K/V store — much more practical than using rigid database schemas for evolving agent knowledge.

API

ctx = RequestContext.get()

# Write — persists across all requests within the same application
ctx.app_state.set("user_123_preferences", preferences)

# Read — available from any function execution in the same app
prefs = ctx.app_state.get("user_123_preferences")

How it works

• Mirrors the existing request-scoped ctx.state architecture at every layer
• Same 3-phase blob protocol: prepare_read → transfer → commit_write
• Same presigned URL pattern for blob store access
• Different URI namespace: {namespace}/{application}/app_state/{key} (vs {namespace}/{application}/{request_id}/state/{key} for request state)
• Last-writer-wins concurrency (S3 PUT is atomic per-object)

Changes

• SDK interface: ApplicationState class + app_state property on RequestContext
• Proto: App state operation/result messages in function_executor.proto + regenerated stubs
• FE allocation runner: AllocationAppState class, wired into AllocationRunner and AllocationStateWrapper
• HTTP handlers: Base, function-executor, and local handlers for prepare_read/prepare_write/commit_write
• HTTP client: ApplicationStateHTTPClient wired into RequestContextHTTPClient

Note: Requires corresponding indexify repo changes (proto + Rust server/dataplane) for remote mode to work end-to-end.

Test plan

• All existing request state tests pass (no regressions)
• Integration test: set/get within single function
• Integration test: set in one function, read in downstream function
• Integration test: two different requests read/write each other's data (same app)
• Integration test: last-writer-wins overwrite across requests
• Integration test: default values for missing keys