docs(triple): complete Triple Header/Error comments and unit test coverage (#3649)

* docs(triple): add doc comments for header, trailer and error exported APIs

Several exported APIs in the triple protocol lacked doc comments or had
incomplete ones. Add a doc comment for ExtractFromOutgoingContext
describing what it returns and how the framework uses it internally:
on the client side to populate request headers, and on the server side
to merge handler-set headers into response trailers. Augment the
SetHeader, SetTrailer and SendHeader comments to state that they return
a CodeInternal error when called outside a Triple handler context. Add
a doc comment for Error.Error describing its format.

Signed-off-by: lizining <lizining1231@outlook.com>

* test(triple): add boundary tests for header, trailer and error APIs

Add 13 tests covering previously untested branches and edge cases:

header_test.go (8 tests):
- DecodeBinaryHeader: empty, unpadded, padded and naturally-aligned inputs
- ExtractFromOutgoingContext: nil return when no headers set, correct
  return when headers exist
- NewOutgoingContext: second call replaces existing headers instead of
  merging
- AppendToOutgoingContext: panics on odd number of kv arguments
- SetHeader/SetTrailer: normal path merges headers into the response
  buffer
- SetHeader/SetTrailer outside a handler context: returns CodeInternal

error_test.go (5 tests):
- NewWireError + IsWireError: wire, plain, non-triple and wrapped cases
- NewErrorDetail with *anypb.Any: skips double wrapping
- ErrorDetail.Bytes: returns a copy, mutation does not affect internal
  state
- wrapIfContextError: Canceled, DeadlineExceeded, already-coded, plain
  error and nil
- wrapIfUncoded: nil, already-coded, context.Canceled and plain error

Signed-off-by: lizining <lizining1231@outlook.com>

* style(triple): silence staticcheck SA5012 in the odd-kv panic test

The test deliberately passes a single kv argument to
AppendToOutgoingContext to verify it panics on an odd number of
arguments. staticcheck SA5012 flags the variadic call as having an odd
element count, which fails the CI lint. Annotate the call with
//nolint:staticcheck, following the existing convention in
handler_compat.go and handler_stream_compat.go.

Signed-off-by: lizining <lizining1231@outlook.com>

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Signed-off-by: lizining <lizining1231@outlook.com>