test: add focused behavior coverage for SessionRepositoryAdapter

[Adesh-tech09]

Closes #968

What changed

• Adds src/auth/login/sessionRepositoryAdapter.test.ts — the dedicated fixture that src/auth/login/sessionRepositoryAdapter.ts was missing.
• Production code is untouched: the existing public contract is preserved exactly.

Covered behavior

Case	Assertion
Interface conformance	SessionRepositoryAdapter stays assignable to the login module's SessionRepository interface
Field mapping	camelCase input maps to { id, user_id, token_hash, expires_at }
No extra keys	Unknown properties are not forwarded to the DB layer
Date identity	The exact Date instance is passed through
Return contract	Resolves to undefined; the persisted row is discarded
Await semantics	Adapter does not resolve before the DB write settles
DB failures	Errors propagate by identity (no swallowing, no re-wrapping)
Isolation	A failed write does not poison the following call
Boundary/abuse input	Empty-string ids and a non-Date expiry are forwarded verbatim for the DB layer to reject
Concurrency	Two parallel calls produce two independent, correctly-mapped invocations

Validation

npx jest src/auth/login/sessionRepositoryAdapter.test.ts src/auth/oidc/oidcRoute.test.ts --runInBand --coverage \
  --collectCoverageFrom='src/auth/login/sessionRepositoryAdapter.ts' \
  --collectCoverageFrom='src/auth/oidc/oidcRoute.ts' --coverageReporters=text

PASS src/auth/login/sessionRepositoryAdapter.test.ts
PASS src/auth/oidc/oidcRoute.test.ts
Test Suites: 2 passed, 2 total
Tests:       68 passed, 68 total

sessionRepositoryAdapter.ts | 100% stmts | 100% branch | 100% funcs | 100% lines

• npx jest src/auth --runInBand → 260 passed; the single failure in src/auth/register/__tests__/roundtrip.test.ts is pre-existing on master and untouched by this PR.
• npx eslint src/auth/login/sessionRepositoryAdapter.test.ts → clean.

Security notes / assumptions

• The adapter is a deliberate pure pass-through: it performs no local validation or normalization. All validation and constraint enforcement stays in the DB repository/database, so a hostile payload cannot be silently "cleaned" into something the persistence layer would accept — the tests lock in that pass-through semantics.
• No credentials, sessions or network were exercised; every DB collaborator is a mock, so the suite is deterministic and offline.
• The test asserts the adapter never returns the persisted row, keeping callers decoupled from DB row shape (including token_hash).

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