🚀 Release v2.5.5 — PDF forwarding, configurable commit prompts, avatar fix (#144)

## Change Log / Overview

Merges `main` into `dev/v2.5.5` and prepares the v2.5.5 release. This PR
brings in the following changes already merged to `main` via individual
PRs, plus release bookkeeping:

### 🚀 Features
- **📄 Send PDFs as `file.file_data` instead of dropping them (#89,
#140)** — Binary PDF content parts are now encoded as `{ type: "file",
file: { filename, file_data } }` data URIs and forwarded on both chat
and `/responses` requests. Azure rejects `application/pdf` inside
`image_url`, so PDFs use `file.file_data` while images keep `image_url`
— the only shape accepted across Azure, Vertex Gemini, and
Bedrock/Vertex Claude.
- **⚙️ Configurable commit message prompts (#142, #143)** — New
`litellm-connector.commitSystemPromptOverride` and
`litellm-connector.commitMessagePromptOverride` settings customize the
git commit message system/style prompts, with a `resolvePrompt()` helper
falling back to built-in defaults when unset/whitespace-only.

### 🐛 Fixes
- **🖼️ Fix broken `@amwdrizz` contributor avatar (#139, #141)** —
Swapped the login-based avatar URL for the numeric user id across
`.all-contributorsrc`, `README.md`, and `CONTRIBUTORS.md`.

### 🧹 Chores
- Added `PR Reviewer` custom agent, `/pr` prompt,
`code-review.instructions.md`, and a post-edit format hook.
- Added `.gitattributes` to normalize line endings to LF; fixed Windows
`spawn` for the test runner.
- Added `LiteLLM_API-v1.97.json` API schema reference (renamed prior
schema to `LiteLLM_API-v1.82.3.json`).
- **Release prep**: bumped version `2.5.5-dev1` → `2.5.5`, added
`CHANGELOG.md` entry for `2.5.5`, and updated both `README.md` /
`README.marketplace.md` "What's New" sections.

## Related Issues, Builds, Pipeline Runs, etc.

- #89
- #139
- #142

## Pull Request Pre-check

- [x] Linting Validation Passed — 0 errors, 31 pre-existing warnings
(baseline unchanged)
- [x] Formatting Validation Passed
- [x] Unit Testing Passes — 1057 passing; S 91.31% / B 82.60% / F 89.03%
/ L 91.31%
- [x] Documentation Updated — `CHANGELOG.md`, `README.md`,
`README.marketplace.md`