fix(compression): keep tool_result blocks first when aging annotates a turn

[ntdatt812]

What

replaceTextContent() prepends the aging annotation when the message has no
text block to replace:

if (!replaced) {
  return { ...msg, content: [{ type: "text", text: newText }, ...msg.content] };
}

A user turn that answers a tool_use with a single tool_result block hits
exactly that branch, so aging rewrites it to ["text", "tool_result"]. The
Anthropic Messages API requires the tool_result blocks to lead that turn, so
upstream rejects the entire request:

messages.N: `tool_use` ids were found without `tool_result` blocks immediately after

This PR appends the annotation when the turn carries a tool_result, and keeps
prepending for every other shape, so no existing ordering changes.

Verifying the report first (#12890)

Confirmed on the tip, at the call site rather than from the description:

• progressiveAging.ts::applyAging → setContent → replaceTextContent. A
  tool-result-only turn has extractTextContent(content) === "", so replaced
  stays false and the prepend branch runs.
• Reproduced through applyAging (not by calling the helper directly): a turn at
  distanceFromEnd = 5 lands in the fullSummary tier and comes back as
  ["text", "tool_result"].

The report's suggested one-line fix (append unconditionally) would also move the
annotation for messages that carry no tool_result — images, for one — so the
condition is scoped to the shape that upstream actually constrains.

Test

tests/unit/compression/aging-tool-result-order-12890.test.ts

1. Wiring: runs a real 8-message conversation through applyAging() and asserts
   the aged turn's block order is ["tool_result", "text"] and that the trailing
   block is the [COMPRESSED:aging:…] annotation. It goes through the aging entry
   point, not replaceTextContent in isolation.
2. Guard: a turn holding only an image block still gets ["text", "image"].

Mutation

mutation	result
!replaced branch back to the unconditional prepend (the shipped bug)	✖ keeps tool_result first through applyAging — got ["text","tool_result"]
!replaced branch to an unconditional append (the report's suggestion)	✖ still puts the annotation first when the turn carries no tool_result

Each mutation kills a different test, so neither branch of the new condition is
unpinned.

Regression

The 7 test files that exercise messageContent.ts / progressiveAging.ts /
aggressive.ts — aggressive-fidelity, aggressive, compression-aggressive,
golden-eval, progressiveAging, compression-aggressive-spare-last-user, plus
the new file: 54 pass, 0 fail.

Closes #12890