Fix OTLP gRPC metadata key normalization

[manas-fortifyroot]

Summary

Normalize OTLP/gRPC metadata keys to lowercase before constructing trace,
metrics, and log exporters.

grpcio rejects HTTP-style uppercase metadata keys such as Authorization,
causing exports to fail locally before any request reaches the backend.

Changes

• Add a shared gRPC metadata-normalization helper.
• Apply it to trace, metrics, and log exporters.
• Preserve HTTP exporter header behavior.
• Add regression coverage for all three signal types.

Validation

• tests/test_sdk_initialization.py: 38 passed.
• FortifyRoot modification-notice validation passed.
• git diff --check passed.
• The full localhost system-test suite passed on Python 3.10 and 3.12.

Post-merge

Create and push immutable fork tag fr-v0.52.6.35. Ocelle must vendor
from that tag rather than from a dirty working tree.