Preserve tracing headers during FromHTTPHeaders conversion

[kexiongliu123]

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RELEASE NOTES: This change updates headerMapper.FromHTTPHeaders to preserve tracing headers such as uber-trace-id and uberctx-* even when they are not prefixed with Rpc-Header-. These headers are critical for baggage and trace context propagation in services using the new tracing interceptor. Additionally, unit tests in header_test.go are updated to assert correct preservation and round-trip conversion of both application and tracing headers. This ensures that services like Cinnamon can properly extract context even when the outbound tracing interceptor is disabled.

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:white_check_mark: kexiongliu123
:x: kl943
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