Maintenance: convert 50_ode/{00,40}*.md to .ipynb for consistency

Two files in `50_ode/` are markdown rather than notebooks:

- `50_ode/00_Ordinary_Differential_Equation_Exact_Solution.md`
- `50_ode/40_comparing_with_num_int.md`

Every other lesson in the module (and the curriculum at large) is a `.ipynb`. The two `.md` files break the "every numbered lesson is an executable notebook" pattern, and they are excluded from notebook-execution CI.

## Scope

- Convert each `.md` to `.ipynb` (`jupyter nbconvert --to notebook --from markdown`, or hand-rebuild if the markdown predates a useful structure).
- Preserve the markdown prose as markdown cells; promote any code blocks to code cells.
- Add Colab badges (the existing badge automation will pick them up after merge).
- Verify the new notebooks execute under the standard test runner.

## Why

Consistency: learners arriving at `50_ode/` see a uniform set of notebooks, no special "this lesson is markdown only" exception. CI consistency: the new notebooks become subject to the same execution-as-test guarantee as everything else.

Small, mechanical, low-risk. Good first PR for someone wanting to learn the repo conventions.