bedrock_endpoint() hardcoded the endpoint/host iso building it via the
result from the configured/resolved pipeline.

This change builds the bedrock URL from the resolved api_base instead.
This allows a user to override the Bedrock endpoint through the BEDROCK_API_BASE env variable.
This matches the behaviour of the other domains/llms.

Retry on empty addition

Allow bedrock endpoint override via BEDROCK_API_BASE environment variable

Lets editors that speak the Agent Client Protocol, such as Zed, launch cecli
as an agent over stdin/stdout.

- Prompts drive the normal linear run loop; slash commands are announced
  as available commands and @-mentioned files are added to the chat
- Model output and reasoning stream as message and thought chunks; agent
  tool calls become tool cards, the todo list becomes the plan, and the
  Yield summary is shown as a message
- Confirmations become permission requests that show their question;
  cancel interrupts the running request
- Mode (including Agent YOLO), model and reasoning effort are offered as
  session config options backed by the matching cecli commands; the model
  list adds the chat models of every provider with an API key set, and
  --acp-models narrows it
- MCP servers from the editor (stdio, HTTP, SSE) are connected alongside
  cecli's own
- Sub-agents are reported as ACP subagent sessions (unstable RFD) when the
  client advertises the subagents capability: each gets its own session,
  work state, messages between sessions and its own cancel
- Pagers are disabled for commands, and the per-request token/cost line is
  kept out of the editor

The agent lives in cecli/helpers/server/acp/agent/. Core changes are
InputOutput.tool_call_event and subagent_event hooks (no-ops outside ACP),
a usage-report output type, AgentService.cancel_sub_agent, the --acp and
--acp-models flags and startup wiring in main.py. Cancelling a sub-agent no
longer fails a blocking Delegate or a Yield waiting on it, and is reported
as interrupted even when streaming absorbed the cancel.

…n improper boolean/object frankenstein

…l cross loop RuntimeErrors that deadlock the system

- Mark announced-but-not-run tools failed when a turn ends
- Distinguish caller cancellation from a cancelled sub-agent
- Deep-merge acp-models like other append options
- Give the ACP docs page a unique nav order

The test let startup pick a default model from the environment, so its assertion depended on which API keys were set. Pin it to a fixed model.