sec(oidc): switch JWT signing from PKCS1v15 to ES256

[cristim]

Summary

• Replaces RSA-PKCS1v15 (RS256) with ECDSA P-256 (ES256) across the entire internal/oidc package
• Updates Algorithm constant, Signer interface, LocalSigner, all three KMS signer stubs, the JWKS builder, and all tests
• All tests positively assert ES256 algorithm and EC JWK fields (kty=EC, crv=P-256, x, y) -- regression to RS256 will fail tests

Closes #422

Test plan

• go test ./internal/oidc/... -- 9 tests pass
• go test ./internal/api/... -- handler_oidc_test.go asserts EC JWK fields
• go vet ./... -- clean
• TestLocalSignerMintAndVerify asserts alg == "ES256" and verifies with ecdsa.VerifyASN1
• TestBuildJWKS asserts kty=EC, crv=P-256, presence of x/y, absence of RSA n/e
• TestBuildDiscovery asserts id_token_signing_alg_values_supported == ["ES256"]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 869e16f8-bb92-46b1-a7b1-50d989948c5d

📥 Commits

Reviewing files that changed from the base of the PR and between 4956d66 and 560ba87.

📒 Files selected for processing (8)

• internal/api/handler_oidc_test.go
• internal/oidc/aws_signer.go
• internal/oidc/aws_signer_test.go
• internal/oidc/azure_signer.go
• internal/oidc/gcp_signer.go
• internal/oidc/jwks.go
• internal/oidc/signer.go
• internal/oidc/signer_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/422-wave20

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[cristim]

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