Refactored provider handling in Client to use OrderedMap, improving
consistency and sorting. Added generic type annotations for OrderedMap
and introduced a get_cached helper for cache management. This change
simplifies provider filtering and caching logic for models and info.

This change refactors the chat completion logic by moving it from
init.lua into a new completion.lua module. It also updates mappings
and setup to use the new module, improving code organization and
maintainability. Autocomplete setup is now handled in the completion
module, and prompt listing is extracted to a helper function.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

Previously, the resource preview header displayed the resource name, which
could be missing. This change updates the header to show the resource URI,
providing clearer context. Also, removes unused type annotations for improved
code clarity.

Refactored chat window API to use `get_message(role, cursor)` and
`get_block(role, cursor)` for both last and cursor-closest access.
Removed legacy `get_closest_message` and `get_closest_block` methods.
Updated mappings and documentation to reflect unified API. This
simplifies usage and improves consistency for message/block retrieval
and removal actions.

Adds support for displaying the assistant's reasoning above messages in
the chat UI as virtual text. Reasoning is now streamed and stored
separately from content, and models with reasoning capability are
indicated in the model selector. This improves transparency of model
responses and debugging.

Refactored Chat:get_block to properly select the closest block to the
cursor, considering the role filter and ensuring correct fallback to the
last matching block. This improves accuracy when interacting with chat
blocks in the UI.

Move role strings and plugin name to a dedicated constants module.
Refactor all usages to reference the new constants. This improves
maintainability and reduces duplication across the codebase.

Refactored chat and overlay constructors to remove redundant help argument
and use key_to_info for help and close mappings directly. This improves
clarity and maintainability by centralizing mapping logic and reducing
parameter complexity.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>