chore: (For Discussion) Dynamic runtime detection & block_on abstraction

[scottgerring]

Fixes #2643, stabilising Runtime trait. Note that i've not removed the experimental flags, but will do so when we're happy with an approach. Note that i've not removed the experimental flags, but will do so when we're happy with an approach.

This is a light touch approach to stabilising the runtime trait.
We do this by consolidating Tokio and TokioCurrentThread into a single Tokio runtime type that auto-detects the active Tokio runtime flavour at call time using Handle::try_current() and runtime_flavor():

• Multi-threaded runtime: spawns directly via tokio::spawn
• Current-thread runtime: spawns on a dedicated OS thread with its own Tokio runtime to avoid deadlocks
• No runtime available: falls back to an OS thread with a new Tokio runtime

This eliminates the rt-tokio-current-thread feature flag entirely — users just enable rt-tokio and the right behavior happens automatically.

Additionally adds a block_on method to the Runtime trait, which is also runtime-flavour-aware: on current-thread it uses std::thread::scope to block on a separate thread, avoiding deadlocks. This replaces ad-hoc futures_executor::block_on calls scattered through processors.

this aims to be a minimal, focused change - keeps the existing oneshot-based shutdown signalling.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.2%. Comparing base (d91b847) to head (90aba49).

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/runtime.rs	86.9%	6 Missing ⚠️
.../src/metrics/periodic_reader_with_async_runtime.rs	91.6%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3412   +/-   ##
=====================================
  Coverage   83.1%   83.2%           
=====================================
  Files        128     128           
  Lines      24899   24921   +22     
=====================================
+ Hits       20715   20743   +28     
+ Misses      4184    4178    -6     

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