[CODE HEALTH] fix clang-tidy cppcoreguidelines-pro-type-const-cast warnings

[Anish701]

Fixes #4477

Changes

Resolves all cppcoreguidelines-pro-type-const-cast warnings by eliminating const_cast usage through proper typing.

Note: The methods MultiObserverResult::ForInstrument and MultiObserverResult::HasInstrument have their parameters changed from const ObservableInstrument* to ObservableInstrument*. The previous const qualifier was misleading because there was a const_cast on the pointer. Also, all existing callers already pass non-const pointers. This is not a breaking change to the API. All unit tests pass successfully.

• examples/http: Accept const T& in HttpTextMapCarrier constructor, removing the need to const_cast request headers.
• ext/http/client/curl: Change SetCurlPtrOption to accept const void*, and use mutable std::vector copies for blob data instead of const_cast from string data.
• ext/test/w3c_tracecontext_http_test_server: Copy headers by value instead of const_casting a reference.
• sdk/common/circular_buffer.h: Make PeekImpl() a const method, eliminating the const_cast<CircularBuffer*>(this) in Peek().
• sdk/metrics/multi_observer_result: Change ForInstrumentDouble, ForInstrumentInt64, and HasInstrument to accept ObservableInstrument* (non-const) since the internal map key is non-const. This is an API signature change because the previous const parameter was misleading as mentioned in the note above.

• CHANGELOG.md updated for non-trivial changes - N/A this is a trivial code health change
• Unit tests have been added - N/A
• Changes in public API reviewed - yes mentioned in description

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Anish701 / name: Anish Sinha (4297c36)
• ✅ login: Anish701 / name: Anish701 (d2890d8)

[codecov]

Codecov Report

❌ Patch coverage is 36.84211% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.11%. Comparing base (45f2629) to head (e36cf5b).

Files with missing lines	Patch %	Lines
ext/src/http/client/curl/http_operation_curl.cc	7.70%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4482      +/-   ##
==========================================
- Coverage   83.12%   83.11%   -0.00%     
==========================================
  Files         519      519              
  Lines       20256    20254       -2     
==========================================
- Hits        16835    16832       -3     
- Misses       3421     3422       +1     

Files with missing lines	Coverage Δ
...lude/opentelemetry/metrics/multi_observer_result.h	100.00% <ø> (ø)
...lemetry/ext/http/client/curl/http_operation_curl.h	90.91% <100.00%> (-0.39%)	⬇️
...include/opentelemetry/sdk/common/circular_buffer.h	100.00% <100.00%> (ø)
sdk/src/metrics/multi_observer_result.cc	92.31% <100.00%> (-0.41%)	⬇️
ext/src/http/client/curl/http_operation_curl.cc	60.30% <7.70%> (-0.29%)	⬇️

... and 1 file with indirect coverage changes

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