Replace hardcoded store name strings with constants in request signing

[ChristianPavilonis]

Summary

• Define JWKS_CONFIG_STORE_NAME and SIGNING_SECRET_STORE_NAME constants as the single source of truth for edge-side store names, replacing 6 scattered string literals
• Document the store name vs store ID distinction (edge reads via ConfigStore::open vs Fastly management API writes)
• Add doc comments on KeyRotationManager fields and constructor clarifying which identifier each holds

Closes #399
Related: #396 (production readiness audit)

Changes

File	What changed
crates/common/src/request_signing/mod.rs	Added constants + module-level docs
crates/common/src/request_signing/signing.rs	Replaced 4 hardcoded strings
crates/common/src/request_signing/jwks.rs	Replaced 1 hardcoded string
crates/common/src/request_signing/rotation.rs	Replaced 1 hardcoded string, added doc comments

Test plan

• cargo test --workspace — all tests pass
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• No behavior change — purely mechanical extraction of string literals into constants

[aram356]

Summary

Clean extraction of hardcoded store name strings into module-level constants with good documentation. All CI checks pass.

Non-blocking

♻️ refactor

• Doc-link-only import masked by #[allow(unused_imports)]: SIGNING_SECRET_STORE_NAME imported only for an intra-doc link; use a fully-qualified path instead (rotation.rs:14)

🤔 thinking

• Test literals coincide with new constants: rotation.rs:232 and :246 pass "jwks_store" / "signing_keys" as store IDs (management API), not store names (edge reads) — so they're semantically different from the constants. But the identical strings obscure that distinction. Consider using clearly distinct placeholder IDs (e.g. "test-config-store-id", "test-secret-store-id") or adding a comment clarifying these are management API store IDs, not edge store names.

👍 praise

• Store name vs store ID documentation: The module doc section clearly explains the two identifier concepts — this was the core confusion behind C-1 (mod.rs:6-16)

CI Status

• cargo fmt: PASS
• cargo test: PASS
• vitest: PASS
• format-docs: PASS
• format-typescript: PASS

[ChristianPavilonis]

Both items addressed in a4a5b92:

1. Doc-link-only import — Removed the #[allow(unused_imports)] and the SIGNING_SECRET_STORE_NAME import. The doc comment now uses a fully-qualified path (crate::request_signing::SIGNING_SECRET_STORE_NAME) instead.

2. Test store ID literals — Renamed "jwks_store" / "signing_keys" to "test-config-store-id" / "test-secret-store-id" so it's immediately clear these are management API store IDs, not edge store names. This reinforces the distinction documented in the module-level docs.

All checks pass (fmt, clippy, 465 tests).