🚀 Release v2.5.2 — stabilize reasoning continuity (#136)

## Change Log / Overview

### 🐛 Fixes
- Harden thinking-block serialization and recombination so streamed
reasoning replays safely across model switches.
- Retry once for narrowly classified continuity rejections by removing
only invalid message-level thinking blocks.
- Preserve continuity redaction through context-overflow rebuilds and
keep adaptive reasoning fields synchronized.
- Keep native VS Code model rows free of inline pricing while retaining
pricing in details and metadata.

### 🧪 Tests
- Added regression, live-path, endpoint-fallback, and end-to-end
coverage for reasoning continuity, adaptive effort fallback, streaming
metadata, and model switching.

### 🧹 Chores
- Promoted the extension version from `2.5.2-dev5` to `2.5.2`.
- Updated `CHANGELOG.md`, `README.md`, and `README.marketplace.md`.

## Related Issues, Builds, Pipeline Runs, etc.
- None

## Pull Request Pre-check
- [x] Linting Validation Passed (`npm run lint`; existing warnings only)
- [x] Formatting Validation Passed (`npm run format`)
- [x] Unit Testing Passes
- [x] Documentation Updated

Validation summary:
- `npm run compile` — passed
- `npm run lint` — passed with 31 existing warnings, 0 errors
- `npm run format` — passed
- `npm run test:coverage` — blocked by VS Code/Electron renderer crash
in the container; no tests executed and coverage was `Unknown% (0/0)`

Do not merge, tag, or publish from this workflow. Squash-merge from the
web UI, then tag the release on `main` as instructed.