fix: reject credential proofs if activation epoch is not set

[wadealexc]

Fixes an issue where validators with insufficient balance for activation could verify withdrawal credentials, leading to an issue during Pectra fork processing where their balances would temporarily drop to 0.

The attack vector looks like this:

Pre-Pectra/after PEPE:
1. Validator deposits to beacon chain with 31 ETH
2. Validator verifies withdrawal credentials
3. Validator restakes 31 ETH worth of shares

<EigenLayer slashing is released>

4. Validator begins selling 31 ETH worth of security to an AVS

Pectra Upgrade:
5. Validator's current and effective balances are set to 0 and moved to the pending deposit queue
6. Validator starts a checkpoint but waits before progressing it
7. At some point, the pending deposit queue is processed and the validator's balance is restored.

... time passes ...

8. Validator commits a slashable offense in an AVS
9. Validator completes the earlier checkpoint, providing a balance proof that drops their balance to 0. (Their current beacon chain balance is higher, but the checkpoint was started when their balance was 0)
10. Validator is slashed for 0 shares
11. Once slashing has passed, validator starts/completes another checkpoint, restoring their shares.

[github-actions]

Reading tracefile ./lcov.info.pruned
                                             |Lines      |Functions|Branches  
Filename                                       |Rate    Num|Rate  Num|Rate   Num
================================================================================
[src/contracts/]
core/AVSDirectory.sol                          |77.8%    27|77.8%   9|    -    0
core/AVSDirectoryStorage.sol                   | 0.0%     1| 0.0%   1|    -    0
core/DelegationManager.sol                     |96.4%   196|92.3%  39|    -    0
core/DelegationManagerStorage.sol              | 100%     3| 100%   1|    -    0
core/RewardsCoordinator.sol                    |93.5%   108|83.3%  30|    -    0
core/RewardsCoordinatorStorage.sol             | 0.0%     9| 0.0%   1|    -    0
core/StrategyManager.sol                       |95.2%    83|95.8%  24|    -    0
core/StrategyManagerStorage.sol                | 0.0%     3| 0.0%   1|    -    0
libraries/BeaconChainProofs.sol                | 100%    26| 100%  11|    -    0
libraries/BytesLib.sol                         | 0.0%   160| 0.0%  14|    -    0
libraries/EIP1271SignatureUtils.sol            | 100%     3| 100%   1|    -    0
libraries/Endian.sol                           | 100%     3| 100%   1|    -    0
libraries/Merkle.sol                           |95.0%    40| 100%   5|    -    0
libraries/StructuredLinkedList.sol             | 0.0%    45| 0.0%  19|    -    0
permissions/Pausable.sol                       |95.7%    23|90.9%  11|    -    0
permissions/PauserRegistry.sol                 | 100%    12| 100%   6|    -    0
pods/EigenPod.sol                              | 100%   125|96.2%  26|    -    0
pods/EigenPodManager.sol                       |98.7%    75|85.7%  14|    -    0
pods/EigenPodManagerStorage.sol                | 0.0%     5| 0.0%   1|    -    0
strategies/EigenStrategy.sol                   | 0.0%    10| 0.0%   5|    -    0
strategies/StrategyBase.sol                    |89.7%    39|77.8%  18|    -    0
strategies/StrategyBaseTVLLimits.sol           | 100%    12|83.3%   6|    -    0
token/BackingEigen.sol                         |72.0%    25|50.0%  10|    -    0
token/Eigen.sol                                |38.5%    39|50.0%  12|    -    0
utils/UpgradeableSignatureCheckingUtils.sol    | 0.0%     6| 0.0%   4|    -    0
================================================================================
                                       Total:|72.0%  1078|71.5% 270|    -    0