ci: drop the duplicate proxy_unit_tests letter-shard workflow

tests/proxy_unit_tests/ runs twice on every pull request. The nine
alphabetical shards in test-unit-proxy-legacy.yml cover the same
directory as the twelve semantic shards in test-unit-proxy-db.yml,
and all nine are required checks, so each PR pays for the directory
twice before it can merge.

The semantic shards are a strict superset. Expanding both matrices
against the working tree, the legacy globs collect 58 files while the
semantic shards name all 59: test_model_response_typing is a directory
and matches none of the test_[a-z]*.py patterns, so the legacy lane has
silently skipped it. The semantic workflow also carries its own
assert-shard-coverage guard, which fails if a file under that directory
is not assigned to a shard, so a new file cannot drop out of CI once the
alphabetical fallback is gone.

Verified with .github/scripts/assert_ci_coverage.py: 2380 test files
have a runner both before and after the deletion. Removing
test-unit-proxy-db.yml as well takes the same guard red with 58
orphaned files, which confirms the guard is live and that the semantic
shards, not the legacy ones, are what hold the coverage.

The nine bare contexts this workflow published (auth-and-jwt,
key-generation, proxy-config, proxy-server, proxy-server-extras,
proxy-token-counter, proxy-response-and-misc, proxy-user-auth-and-spend,
proxy-utils) still need pruning from the guard-internal-staging ruleset,
which needs admin rights and is not part of this change

.github/workflows/test-unit-proxy-legacy.yml

@@ -1,106 +0,0 @@
-name: "Unit Tests: Proxy Legacy Tests"
-
-on:
-  pull_request:
-    branches:
-      - main
-      - litellm_internal_staging
-      - litellm_oss_staging
-      - "litellm_**"
-  push:
-    branches:
-      - main
-      - litellm_internal_staging
-
-permissions:
-  contents: read
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
-  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
-
-jobs:
-  test:
-    runs-on: ubuntu-latest
-    timeout-minutes: 20
-    strategy:
-      fail-fast: false
-      matrix:
-        test-group:
-          - name: "auth-and-jwt"
-            path: "tests/proxy_unit_tests/test_[a-j]*.py"
-          - name: "key-generation"
-            path: "tests/proxy_unit_tests/test_[k-o]*.py"
-          - name: "proxy-config"
-            path: "tests/proxy_unit_tests/test_prisma*.py tests/proxy_unit_tests/test_prompt*.py tests/proxy_unit_tests/test_proxy_[c-r]*.py"
-          - name: "proxy-server"
-            path: "tests/proxy_unit_tests/test_proxy_server.py"
-          - name: "proxy-server-extras"
-            path: "tests/proxy_unit_tests/test_proxy_server_*.py tests/proxy_unit_tests/test_proxy_setting_guardrails.py"
-          - name: "proxy-utils"
-            path: "tests/proxy_unit_tests/test_proxy_utils.py"
-          - name: "proxy-token-counter"
-            path: "tests/proxy_unit_tests/test_proxy_token_counter.py"
-          - name: "proxy-response-and-misc"
-            path: "tests/proxy_unit_tests/test_[r-t]*.py"
-          - name: "proxy-user-auth-and-spend"
-            path: "tests/proxy_unit_tests/test_[u-z]*.py"
-
-    name: ${{ matrix.test-group.name }}
-
-    steps:
-      - uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
-        with:
-          persist-credentials: false
-
-      - name: Detect backend-relevant changes
-        id: changes
-        uses: ./.github/actions/detect-backend-changes
-
-      - name: Set up Python
-        uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5.6.0
-        with:
-          python-version: "3.12"
-
-      - name: Set up uv
-        uses: ./.github/actions/setup-uv-with-retries
-        with:
-          version: "0.10.9"
-
-      - name: Cache uv dependencies
-        uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0
-        with:
-          path: |
-            ~/.cache/uv
-            .venv
-          key: ${{ runner.os }}-uv-${{ hashFiles('uv.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-uv-
-
-      - name: Install dependencies
-        if: steps.changes.outputs.decision != 'skip'
-        run: |
-          .github/scripts/uv_sync_with_retries.sh --frozen --group ci --group proxy-dev --extra google --extra proxy --extra semantic-router
-
-      - name: Cache Prisma binaries
-        if: steps.changes.outputs.decision != 'skip'
-        uses: ./.github/actions/cache-prisma-binaries
-
-      - name: Generate Prisma client
-        if: steps.changes.outputs.decision != 'skip'
-        run: |
-          uv run --no-sync prisma generate --schema litellm/proxy/schema.prisma
-
-      - name: Run tests - ${{ matrix.test-group.name }}
-        if: steps.changes.outputs.decision != 'skip'
-        env:
-          TEST_PATH: ${{ matrix.test-group.path }}
-        run: |
-          uv run --no-sync pytest ${TEST_PATH} \
-            --tb=short -vv \
-            --maxfail=10 \
-            -n 2 \
-            --reruns 1 \
-            --reruns-delay 1 \
-            --dist=loadscope \
-            --durations=20