Tighten region regex: reject trailing hyphens and enforce 63-char DNS label limit

[Copilot]

The region validation regex ^[a-z][a-z0-9-]*$ accepted values that produce malformed DNS labels in {region}.login.microsoft.com — specifically, trailing hyphens (e.g. eastus-) and strings exceeding 63 characters.

Changes

• msal/region.py: Tightens _VALID_REGION_RE to require an alphanumeric ending character and cap total length at 63 chars; adds _validate_region() helper used in _detect_region and _detect_region_of_azure_vm
• msal/application.py: Calls _validate_region() on region_to_use before constructing the regional authority URL
• tests/test_region.py: New test module covering valid/invalid cases, including the previously-unguarded trailing hyphen and >63-char inputs

# Before — both accepted, both produce malformed hostnames
_VALID_REGION_RE = re.compile(r"^[a-z][a-z0-9-]*$")
_validate_region("eastus-")   # → "eastus-"   (trailing hyphen)
_validate_region("a" * 64)    # → "aaa...aaa" (64 chars)

# After
_VALID_REGION_RE = re.compile(r"^[a-z]([a-z0-9-]{0,61}[a-z0-9])?$")
_validate_region("eastus-")   # → None (rejected, warning logged)
_validate_region("a" * 64)    # → None (rejected, warning logged)