chore: resolve latest cargo-shear findings

[CathalMullan]

Resolves the latest cargo-shear reported issues.

Will unblock #3272.

> cargo shear --expand
shear/misplaced_dependency

  × misplaced dependency `ctor`
    ╭─[opentelemetry-otlp/tests/integration_test/Cargo.toml:18:1]
 17 │ anyhow = { workspace = true }
 18 │ ctor = { workspace = true }
    · ──┬─
    ·   ╰── only used in dev targets
 19 │ uuid = { workspace = true, features = ["v4"] }
    ╰────
  help: move this dependency to `[dev-dependencies]`

shear/misplaced_dependency

  × misplaced dependency `uuid`
    ╭─[opentelemetry-otlp/tests/integration_test/Cargo.toml:19:1]
 18 │ ctor = { workspace = true }
 19 │ uuid = { workspace = true, features = ["v4"] }
    · ──┬─
    ·   ╰── only used in dev targets
 20 │ tracing-subscriber = { workspace = true, features = ["env-filter","registry", "std", "fmt"] }
    ╰────
  help: move this dependency to `[dev-dependencies]`

shear/misplaced_dependency

  × misplaced dependency `opentelemetry-appender-tracing`
    ╭─[opentelemetry-otlp/tests/integration_test/Cargo.toml:24:1]
 23 │ [target.'cfg(unix)'.dependencies]
 24 │ opentelemetry-appender-tracing = { path = "../../../opentelemetry-appender-tracing", default-features = false}
    · ───────────────┬──────────────
    ·                ╰── only used in dev targets
 25 │ opentelemetry-otlp = { path = "../../../opentelemetry-otlp", default-features = false }
    ╰────
  help: move this dependency to `[target.'cfg(unix)'.dev-dependencies]`

shear/misplaced_optional_dependency

  ⚠ misplaced optional dependency `serde_json`
    ╭─[opentelemetry-proto/Cargo.toml:62:1]
 61 │ serde = { workspace = true, optional = true, features = ["serde_derive", "std"] }
 62 │ serde_json = { workspace = true, optional = true }
    · ─────┬────
    ·      ╰── only used in dev targets
 63 │ const-hex = { workspace = true, optional = true }
    ╰────
  help: remove the `optional` flag and move to `[dev-dependencies]`

Advice: 
  ☞ removing an optional dependency may be a breaking change

shear/summary

  ✗ 3 errors
  ⚠ 1 warning

Advice:
  ☞ run with `--fix` to fix 3 issues
  ☞ to suppress an issue within a package
   ╭─[Cargo.toml:2:12]
 1 │ [package.metadata.cargo-shear]
 2 │ ignored = ["crate-name"]
   ·            ──────┬─────
   ·                  ╰── add the crate name here
   ╰────
  ☞ to suppress an issue across a workspace
   ╭─[Cargo.toml:2:12]
 1 │ [workspace.metadata.cargo-shear]
 2 │ ignored = ["crate-name"]
   ·            ──────┬─────
   ·                  ╰── add the crate name here
   ╰────

Changes

• Removed unused dependencies.
• Move dependencies only used in tests to dev-dependencies.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.7%. Comparing base (65969fd) to head (3b4752c).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3279   +/-   ##
=====================================
  Coverage   80.7%   80.7%           
=====================================
  Files        129     129           
  Lines      23210   23210           
=====================================
  Hits       18748   18748           
  Misses      4462    4462           

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[Copilot]

Pull request overview

This PR resolves cargo-shear findings by moving misplaced dependencies to their appropriate sections in the Cargo manifests. The changes ensure dependencies only used in test code are properly categorized as dev-dependencies.

Key changes:

• Removed serde_json from optional dependencies and the with-serde feature in opentelemetry-proto, moving it to dev-dependencies where it's actually used (in test files only)
• Moved ctor and uuid from regular dependencies to dev-dependencies in the integration test package, as they're only used in test files
• Moved opentelemetry-appender-tracing to platform-specific dev-dependencies, as it's only used in Unix-specific test files

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
opentelemetry-proto/Cargo.toml	Removed serde_json from the with-serde feature and optional dependencies; it remains in dev-dependencies where it's used for JSON serialization tests
opentelemetry-otlp/tests/integration_test/Cargo.toml	Moved ctor, uuid, and opentelemetry-appender-tracing to dev-dependencies sections, properly categorizing test-only dependencies

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