no longer short/long running

[wilsonwang371]

This pull request updates documentation to reflect recent changes in the task execution model, runtime behavior, and UI test structure. The most significant updates clarify that tasks are now treated as unified executable units with invocation modes, improve the explanation of how missing tasks are fetched and materialized on-demand, and update WASM runtime lifecycle and artifact handling. Additionally, the UI test guides are revised for directory changes and improved accessibility notes.

Task Execution Model & Scheduling

• Added new documents task-execution-model-en.md and task-execution-model-zh.md describing the unified request/response model for tasks, removing distinctions between "long running" and "short running" tasks, and shifting runtime strategies to invocation parameters. [1] [2]
• Updated scheduling and placement documentation to remove the execution_kind field from placement requests and examples, and clarified that Spearlet nodes now fetch missing tasks/artifacts from SMS on-demand, improving flexibility and reliability. [1] [2] [3] [4] [5] [6] [7] [8]

Runtime Behavior Documentation

• Updated both English and Chinese runtime instance communication flow docs to clarify that if a task is missing locally, it is fetched and materialized from SMS before execution, and added notes on runtime-specific semantics for instance startup. [1] [2] [3] [4]
• Revised WASM runtime usage docs (both languages) to clarify lifecycle steps, artifact URI handling, and how module bytes are validated and loaded, including new details about wasmedge worker startup. [1] [2] [3]

UI Test Guide Updates

• Updated UI test guide directories from spear-next/ui-tests to spear/ui-tests in both languages, and removed references to "Execution Kind" select mirrors for test stability, focusing on "Executable Type" for accessibility and automation reliability. [1] [2] [3] [4] [5] [6]

Documentation Index Additions

• Added new entries for "Task Execution Model" and "WASM Runtime Usage" (English and Chinese) to the documentation index for easier navigation. [1] [2]

Minor Corrections

• Updated function name examples in invocation comparison docs for consistency. [1] [2]
• Simplified task list columns in SMS Web Admin redesign docs (both languages) by removing "execution kind". [1] [2]