Migrate quickstart from Functions Core Tools v4 to Functions CLI v5 (func5)

[paulyuk]

What

Move the documented local-dev path from Azure Functions Core Tools v4 to the new Azure Functions CLI v5 preview, installed alongside any existing v4 as func5. This shrinks the local-dev story from three terminals to two and removes the separate Azurite install/start.

Why

The current README:

• Tells users to brew install azure-functions-core-tools@4 + npm install -g azurite + run Azurite in its own terminal.
• Calls v5 out as 'not yet compatible' (the early v5 previews couldn't load the Preview extension bundle, #5309).

That bundle issue is fixed in bundles@4.42.0-preview.2 (shipped via func5 setup --features python), and v5 auto-manages Azurite (func run / func start starts and stops it for you unless you pass --no-azurite). So the v4 + separate Azurite recipe is now strictly more work for new users.

What changed

File	Change
README.md	Prerequisites: drop the 'v4 only, v5 not compatible' line. Quickstart step 1: replace the brew + npm install with the official v5 install pattern (download v5 preview tar.gz/zip, alias as func5, run func5 setup --features python once). Quickstart step 3: collapse 3 terminals to 2 (Azurite goes away).
docs/troubleshooting.md	Remove 'Stay on v4' and the Azurite 'connection refused' / 'InvalidHeaderValue' entries (v5 handles Azurite). Add a self-fix entry that points at the extension bundles workload. New 'Still using v4' section documents the fallback path for users who can't / don't want to add v5 yet.
chat.py, function_app.py, infra/scripts/*.sh, infra/scripts/*.ps1, docs/configuration.md	All user-visible uv run func start references become uv run func5 start. Hydrate scripts hint at v5 auto-Azurite.

Test recipe

On a clean machine:

# Quickstart step 1 (Apple Silicon shown; -osx-x64-, -linux-x64-, or -win-x64- elsewhere)
mkdir -p ~/.azure-functions/v5 && cd ~/.azure-functions/v5
curl -LO https://github.com/Azure/azure-functions-core-tools/releases/download/v5.0.0-preview.2/func-osx-arm64.tar.gz
tar -xzf func-osx-arm64.tar.gz
mkdir -p ~/.local/bin && ln -sf ~/.azure-functions/v5/func ~/.local/bin/func5
# fresh shell so PATH picks up ~/.local/bin
func5 --version            # -> 5.0.0-preview.2+...
func5 setup --features python

# Quickstart steps 2–3
azd provision
./infra/scripts/hydrate-local-settings.sh
uv run func5 start         # one terminal, Azurite started for you
uv run python chat.py      # other terminal

Notes for reviewers / followups

• This is off main, separate from README & usability fit and finish #17 (README usability) and infra: bump default modelCapacity 50 → 150 (avoid day-one 429s) #18 (capacity bump). Both add lines this PR rewrites; whichever lands second will need a tiny rebase.
• Once v5 reaches GA, the 'Still using v4' section should be removed and the install step replaced with a brew formula / aka.ms one-liner.
• I left the v5 download URLs pinned at v5.0.0-preview.2. We can swap to a 'latest preview' URL once Microsoft publishes one (currently the GitHub Releases page is the source of truth).
• The func5 alias is intentional: it keeps the v4 func users have today out of the way. The official v5 binary IS func; the alias is just for coexistence.