Fix whitespace-only text block from orphaned CacheBreakpoint

[bhavyaus]

Problem

Fixes microsoft/vscode#305956

Root Cause

In rawContentToAnthropicContent(), the CacheBreakpoint handler's else branch (when no preceding block exists):

// Old code
convertedContent.push({ type: 'text', text: ' ', cache_control: { type: 'ephemeral' } });

Fix

Instead of creating an invalid placeholder, defer the cache_control to the next cacheable content block that gets added. If no subsequent block exists, silently drop it — there's nothing to cache anyway.

• pendingCacheControl flag tracks orphaned breakpoints internally
• After each content block is pushed, attach any pending cache_control to it
• No change to the function's return type or callers
• No prompt cache regression: the "defer forward" behavior actually improves cache placement (cache covers real content, not a useless space token)

Test Coverage

• New test: cache breakpoint before content defers cache_control to next block — verified it fails on old code (2 blocks including whitespace placeholder) and passes on new code (1 block with correct cache_control)
• Updated test: cache_control-only tool content — orphaned breakpoint with no content is now silently dropped (impossible in practice since addCacheBreakpoints only targets messages with content)
• All 31 existing tests pass

[Copilot]

Pull request overview

Fixes an Anthropic Messages API validation error triggered when prompt pruning leaves an orphaned CacheBreakpoint, which previously produced an invalid whitespace-only text block.

Changes:

• Update rawContentToAnthropicContent() to defer orphaned cache_control forward to the next cacheable content block (or drop it if none appears).
• Update/extend unit tests to cover the new defer-forward behavior and the “drop if no content” case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/platform/endpoint/node/messagesApi.ts	Replaces whitespace placeholder block with a pending/deferred cache_control mechanism to avoid invalid Anthropic text blocks.
src/platform/endpoint/test/node/messagesApi.spec.ts	Updates the cache_control-only tool test expectation and adds a new test verifying deferred cache_control attachment to the next block.