chore(deps): bump OpenTelemetry SDK to v1.44+ and Go to 1.25 (prereq for envcar)

[tobert]

Prerequisite for #31 (mirrors equinix-labs/otel-cli#390).

go.opentelemetry.io/contrib/propagators/envcar@v0.69.0 declares:

• go 1.25.0
• go.opentelemetry.io/otel v1.44.0
• go.opentelemetry.io/otel/trace v1.44.0

We are currently pinned at otel v1.27.0 and go 1.21 (toolchain go1.22.4).

Scope

• Bump go.opentelemetry.io/otel, otel/sdk, otel/trace, otel/metric 1.27.0 -> 1.44.0.
• Review otel/semconv (currently v1.17.0 in otlpclient/otlp_client.go) for breaking moves; bump if warranted.
• Bump the go directive / toolchain to 1.25.
• Check ripple into the hand-rolled OTLP gRPC/HTTP exporters and go.opentelemetry.io/proto/otlp (currently v1.1.0).
• go mod tidy, go vet, then rebuild and run the full functional harness (go build && go test && go test ./...) — the harness execs the binary and validates real OTLP export, so it's our real regression net for an SDK bump.

Acceptance

• All functional + unit tests green against the bumped SDK.
• No behavior change to span emission, exporters, or CLI surface — this PR is deps-only.