Clarify environment carrier key normalization behavior (open-telemetry#5102)

Towards
open-telemetry#5040

## Changes

Clarifies the environment variable propagation carrier key normalization
requirements:

- `Set` writes values using normalized key names.
- `Get` normalizes both the requested propagator key and carrier keys
before matching.
- `Keys` returns normalized key names.

## Motivation

The existing text defined how key names are normalized, but it did not
explicitly say where that normalization applies. This PR clarifies that
normalization is part of the carrier behavior for writing, reading, and
listing keys.

This follows the robustness principle: carriers write the normalized
representation, while getters can still accept a non-normalized
environment variable key if it normalizes to the requested propagation
key. For example, a non-normalized `traceparent` environment variable
can still be used by the propagation getter when matching `TRACEPARENT`.

This retrospects
open-telemetry#4944 (comment)
now that implementations are in place.

## Implementation references

- Go:
open-telemetry/opentelemetry-go-contrib#8761
- .NET: open-telemetry/opentelemetry-dotnet#7174
- Java: open-telemetry/opentelemetry-java#8233
- Python:
open-telemetry/opentelemetry-python#5119
- Swift:
open-telemetry/opentelemetry-swift-core#47

Related Java discussion:
open-telemetry/opentelemetry-java#8233 (comment)

Author: pellared

CHANGELOG.md

@@ -9,6 +9,10 @@ release.
 
 ### Context
 
+- Clarify that environment variable propagation carriers normalize requested
+  keys, carrier keys, and returned keys.
+  ([#5102](https://github.com/open-telemetry/opentelemetry-specification/pull/5102))
+
 ### Traces
 
 ### Metrics

specification/context/env-carriers.md

@@ -66,6 +66,21 @@ Environment variable names used for context propagation:
     (`_`) with an underscore (`_`),
   - prefixing the name with an underscore (`_`) if it would otherwise start with
     an ASCII digit.
+- MUST be normalized consistently by `Get`, `Set`, and `Keys` operations:
+  - when injecting context, the carrier `Set` operation MUST write values using
+    the normalized form of the key provided by the propagator,
+  - when extracting context, the carrier `Get` operation MUST normalize the key
+    requested by the propagator and the key names present in the carrier before
+    matching them. If multiple carrier key names normalize to the same key name,
+    the carrier is ambiguous and in that case, the value returned by `Get` is
+    unspecified,
+  - when listing keys, the carrier `Keys` function MUST return normalized key
+    names.
+
+For example, if a propagator requests the key `x-b3-traceid`, the environment
+variable carrier MUST match it to the carrier key `X_B3_TRACEID`. It MUST also
+match it to a carrier key such as `x-b3-traceid`, because both key names
+normalize to `X_B3_TRACEID`.
 
 > [!NOTE]
 > This normalization is consistent with the environment variable naming rules