feat(lambda-lite): detect Lambda Lite environment and write mini agent sentinel file

[litianningdatadog]

https://datadoghq.atlassian.net/browse/SVLS-8571

What does this PR do?

Adds Lambda Lite detection to datadog-serverless-compat and uses it to write a sentinel file on startup that signals co-located Node.js tracers to switch transport modes.

• Introduces is_lambda_lite() in http_utils, which reads AWS_LAMBDA_INITIALIZATION_TYPE and returns true when the value is "native-http" (the value Lambda Lite sets to distinguish itself from standard on-demand invocations).
• On startup, MiniAgent writes an empty sentinel file to /tmp/datadog/mini_agent_ready when running in Lambda Lite mode. dd-trace (Node.js) polls this path (via DATADOG_MINI_AGENT_PATH in constants.js) to decide whether to use
  AgentExporter (HTTP to :8126) instead of LogExporter (stdout).
• Tightens the release profile: lto = "fat" (full cross-crate LTO), strip = "symbols" (explicit symbol stripping), and panic = "abort" (eliminates unwinding code for a smaller binary).

Motivation

Lambda Lite (web function / snap-start mode) runs as a long-lived HTTP server rather than a short-lived invocation handler. The standard Lambda Extension path (/opt/extensions/datadog-agent) is not available in this environment, so dd-trace has no automatic signal that a local agent is present and defaults to shipping traces via stdout. Writing the sentinel file gives dd-trace the signal it needs to switch to the more efficient HTTP path without requiring any customer configuration.

Additional Notes

• /opt is read-only in Lambda Lite. /tmp/datadog/ is writable and is pre-created by the serverless-compat JS layer before this binary is spawned, so no directory creation is needed here.
• The detection logic is intentionally a pure function of the env var string (is_lambda_lite_from_env) to keep it fully unit-testable without spawning processes or touching the filesystem.
• Release profile changes are Lambda Lite–motivated (minimising cold-start binary size) but apply to all targets.

Describe how to test/QA your changes

• Unit tests for all four AWS_LAMBDA_INITIALIZATION_TYPE states (native-http, on-demand, empty string, unset) are added in http_utils.rs and can be run with: cargo test -p datadog-trace-agent test_is_lambda_lite
• End-to-end: deploy datadog-serverless-compat in a Lambda Lite environment and confirm /tmp/datadog/mini_agent_ready exists after startup, and that a Node.js app instrumented with dd-trace sends spans via HTTP to :8126 rather than stdout.