fix(room): report a failed run beside its reply with the run's own notice

[91jaeminjo]

Summary

PR 1 of #602 (thread replay parity): failure notice beside the reply.

A run that failed after replying used to show a red ErrorMessage row under
the reply. That row knew only the error text, so it offered no way to view or
add the run's note, and it read as a fresh error on every reload. Now the run
reports itself with its own outcome tile, the same muted notice every other
outcome uses:

[N events] [Thinking]          band, on the reply
Assistant
  <reply>
ⓘ Run failed: <detail>        the run's own outcome tile
  View or add a note
  <time>
  [copy]

Red stays where it means "this just happened": the live, dismissible banner
above the input, unchanged.

Changes

• lay_out_timeline.dart: _guaranteeATilePerRun places the run's
  NoResponseTile after the reply instead of minting an ErrorMessage, and
  returns the ids it placed beside a reply. RenderedTile gains
  besideReply. _placeBands uses one hostsBand rule, so a tile beside a
  reply never hosts a band, and a band keyed by its id reads as unclaimed and
  goes to the run's last response.
• NoResponseTileWidget / MessageTile: take besideReply (required, one
  caller each). Beside a reply the tile shows only the reason, the note
  button, the time and the copy button, with no second "Assistant" label.
  With no error text it says "Run failed", because "Run failed without a
  response" would contradict the reply above it. Whitespace-only text
  counts as no text.
• Every failed notice with error text, beside a reply or standing in for
  its run, has a copy button that copies the error text without the
  "Run failed:" label.
• runErrorMessageId and the code that minted it are removed.
  ErrorMessage remains for the pre-run RUN_ERROR only.
• Doc comments that described the old row are corrected
  (run_ending.dart _park, the NoResponseTile class doc, layout docs).

Breaking / behaviour notes

• Library consumers: runErrorMessageId is no longer exported from
  soliplex_client or soliplex_agent (CHANGELOG).
• A thread whose last seen message was the old red row misses the "New
  messages" divider once after updating: its saved position names
  run-error-<run>, which no tile carries any more. New positions are
  saved from the same layout the screen draws, so the divider works for
  them (CHANGELOG).

Tests

• Red on main first: the layout puts the notice after the reply, not an
  ErrorMessage. The timeline renders the failure through the notice
  under a single author label.
• Band placement uses the shape both live and reload produce. I confirmed
  that shape against the real producers: a whitespace response after a
  tool result, then a failure.
• The owed outcome (a run with a reply and work no message spoke for)
  stays off the beside path and keeps its band.
• Widget tests cover:
  • the beside notice hiding band, phase, thinking and author;
  • the wording with and without detail;
  • the copy button in both placements.
• Updated the tests the plan lists:
  • lay_out_timeline_test;
  • derived_timeline_integration_test (both failure tests);
  • live_reload_parity_test (record destructuring);
  • message_tile_test and no_response_tile_widget_test (new
    parameter).
• Removed the tests in this change that repeated another test's input or
  passed without the change. Each of the rules above was checked by
  breaking it in production code; a test went red every time.

Test Plan

• flutter analyze --fatal-infos: no issues
• App tests (--exclude-tags golden): 2961 pass
• soliplex_client 1848 and soliplex_agent 574 pass
  (--exclude-tags integration)
• markdownlint on CHANGELOG.md
• Driven in Chrome against the fault-injection backend, live and after
  switching threads and back:
  • runerror: band on the reply, muted notice, note button, copy (copies
    "Injected failure"), red banner live.
  • Whitespace-only error text: "Run failed", no copy button.
  • cutoff: live, the partial reply plus the notice beside it. Reload
    still drops the partial reply (unchanged; PR 4 of Thread-history replay duplicates the live run path's citation and drop handling #602), so the
    notice stands in with band, note and copy.
  • Normal run: no notice.
  • RAG in the local chat room: reply, band and 2 sources identical
    live and reopened.

Found while testing, not changed here

• ag_ui's decoder rejects a RUN_ERROR with an empty message
  (requireNonEmpty), so such a run reads as cut off, not failed.
• The red banner renders whitespace-only error text as a blank line.
• An undated RUN_ERROR notice has no time live, and on reload is dated at
  run start (before its reply). PRs 4–5 of Thread-history replay duplicates the live run path's citation and drop handling #602 fix this.
• In the timeline, each tile's text merges into one semantics node labelled
  "Copy". This applies to replies and the pre-run row as well, so it is not
  new here.
• thread_view_state_test's "a send from a disposed view does not throw"
  still prints its known "read after disposed" warning.

Related Issues

Part of #602

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