feat(primitives): add runtime service identity types

[josecelano]

Summary

Defines the tracker-owned canonical runtime identity foundation for #2036.

• Adds ServiceRole and ConfigurationInstanceId to torrust-tracker-primitives.
• Derives existing HTTP, REST API, and UDP health-check labels directly from ServiceRole while preserving the tracker_rest_api response value.
• Documents and tests typed, role-qualified, zero-based configuration instance identity independent of socket addresses.
• Clarifies Fix duplicate port-zero tracker instance bootstrap #2035's two-phase delivery: bootstrap propagation follows Define canonical runtime service identity #2036; Migrate runtime service registry metadata #2041 and Normalize per-instance event metrics policy #2039 remain prerequisites for final verification and closure.

Validation

• Focused package tests for primitives, HTTP server, REST API server, and UDP server.
• Pre-commit checks.
• Pre-push checks, including nightly workspace checking/documentation and the full stable test suite.

Scope

This PR deliberately does not implement duplicate-port-zero bootstrap propagation (#2035), registry metadata/query APIs (#2041), or event-metrics normalization (#2039).

Closes #2036

[Copilot]

Pull request overview

This PR establishes tracker-owned, canonical runtime identity types in torrust-tracker-primitives (service role + role-qualified configuration instance identity) to serve as the shared foundation for upcoming bootstrap/registry/metrics follow-ups (#2035, #2041, #2039). It also removes ad-hoc health-check “type string” constants in server launchers and derives those labels from the new canonical ServiceRole, preserving the existing tracker_rest_api value.

Changes:

• Added ServiceRole (serde-stable role identifiers + as_str/Display) and ConfigurationInstanceId (role + zero-based index) with unit tests.
• Updated HTTP, REST API, and UDP health-check job construction to use ServiceRole identifiers instead of local TYPE_STRING constants.
• Updated issue specifications (#2036, #2035) with approved design decisions, progress, and updated tracking.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/primitives/src/service_role.rs	Introduces canonical service-role enum with stable identifiers and tests.
packages/primitives/src/configuration_instance_id.rs	Introduces role-qualified, zero-based configuration instance identity with tests.
packages/primitives/src/lib.rs	Exposes the new identity types via module exports and re-exports.
packages/axum-http-server/src/server.rs	Uses ServiceRole to label the HTTP tracker health-check job.
packages/axum-rest-api-server/src/server.rs	Uses ServiceRole to label the REST API health-check job (preserving tracker_rest_api).
packages/udp-server/src/server/launcher.rs	Uses ServiceRole to label the UDP tracker health-check job.
docs/issues/open/2036-add-runtime-service-registry-metadata/ISSUE.md	Records approved identity design decisions and marks implementation/verification as done.
docs/issues/open/2035-fix-duplicate-port-zero-tracker-instance-bootstrap/ISSUE.md	Clarifies phased delivery plan and updates tracking based on the #2036 handoff.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.57%. Comparing base (dc5fa68) to head (7a07fff).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2042   +/-   ##
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  Coverage    81.57%   81.57%           
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  Files          344      344           
  Lines        24645    24653    +8     
  Branches     24645    24653    +8     
========================================
+ Hits         20103    20111    +8     
  Misses        4238     4238           
  Partials       304      304           

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[josecelano]

ACK 7a07fff